owww... If you want to catch them at Diamond and Pearl, Darkrai at Newmoon island, Shaymin at Flower paradise, and Arceus in wild

it depends do you have Pearl or Diamond? If you do then you have to use action replay. With this new game Platinum I'm pretty sure you don't have to use action replay.

actually, there IS a way too get shaymin without using action replay or hacks.

The video on how to do so is below.
I think there is a similiar way to get darkrai, but i'm not sure about arceus.

you need to go to the pokemon ceter

Actually, i dont know where you live,but i got shaymin by downloading it from a pokemon download thingy, basically there was a nintendo guy and i you have to do it via Wi-Fi,
look up on the internet if there is a event near you soon, , this happend last week so i dont know if it still goes on, but i you can get shaymin on diamond and pearl, you first have to go to the Jubilife TV station on the 3rd floor and there will be a guy standing with a girl, the guy will ask you a question, and the first thing you answer is "everybody Happy" and the next question you answer "wi-fi Connection" and you get a mystery gift, you will need that mystery gift to download shaymin at a event, i got 2 shaymins both level 50, and once you trade it to platnium you can change it to its sky form, by giveing it a flower from floraroma town...
i hope my information is ok :P

^^
joi

you must have platinum and a kyoger

Well, there used to be a event at Toys'r'Us for those pokemon. But now it has past. Here are the items: Arceus: Azure Flute
Darkrai: Hotel Tiket
Shaymin: Oak's Letter

You could also trade it over Nintendo Wi-Fi at the GTS Station

you need the action replay or you cant get to new moon island or not get creisala

you cant get shaman or darkrie without a action replay or of cores a trade but you can get arces with mistry gift if you know some one who has the arces flute then you can get mistry gift and connect with there ds and they will give you it

To get Darkrai, Shaymin, and Arceus you need to get the national pokedex by completing the game and have the sinnoh pokedex filled with every pokemon

I hav PK diamond and no AR. I hav Cresselia. Im sure u don't NEED AR 4 ne of that. Trust me i caught the 3 lake pokemon without masterballs and i did Cresselia the same way. I've already been to Full Moon Island. All they did with Darkrai was lock the door without a key...literally. You can trade to get that item with sum1 on wifi. Try DS-play.com

THE ONLY THING U HAVE 2 DO IS 2 PRESS THE SKIP BUTTON

if you have platinum you have to do this before september 13th ok at the title screen click mystery gift then click recieve now click nintendo WFC but you have to have internet next you will recieve the hotel ticket then go to canalava town and talk to the sailors son for darkria

Sorry that`s almost impossable trust me I`ve tryed but there is an SI way to do it but its hard.And Platnim version is no diffrent. Anyway action replay is sold in nentendo world in manhattan.

You can catch it on the last floor of vitory road or you do the glitchs and use the voids cause i have done it but can't explain so look on youtube.com
From lucas.k

you can or find this flute that sumons arceus

I think they are having events for platinum which are repeats only for platinum.As for arceus,the event hasn't come out.YET!!!

Mystery gifts are the answer. You can have pearl, diamond or platinum. Go to Jubilife city and into the 3rd floor of the tv station. Talk to the man standing there. After he speaks, say, EVERYONE HAPPY. Then after he talks, say WI-FI CONNECTION. After that, save it, then turn it on again. Choose MYSTERY GIFT from the menu. Then choose GET VIA NINTENDO WFC. Depending when it is, you can get one of the mystery gifts.

you could get dark rai by going to canalave city open the house that was imposible to open the guy will make you go to msleep the room will start flashing you will end up on fullmoon island go into the forest on the island and you will see dark rai.

first u have to find every pokemon in sinnoh and go to calavyne town where u go to iron island and go to the house with the kid and his mom and talk to the mom then go to the sailor right next to the house and u will get to go to a new place u will find cressilia and u have to find the lunar wing and go bac to the house and give it to him and then go to juble life and talk to the reporter on the 3 floor and say everyone happy and then Wi-Fi connection then save your game turn it off and u will find a mystery gift it will be a key to the hotel in calavyn town and open it and fall asleep and u will go into a nightmare and Darkrie will be there

You will have to wait for Japan to release English Pokemon events, which I doubt they will.
Action Replay is definately worth it mind that. My beloved Darkrai (Smokey) is awesome.
But if you have Pokemon Battle Revolution and you have Action Replay, remember that you cannot battle with pokemon like Arceus, Lugia etc. on Wi-Fi. Don't know how all the other people do it.
Hope this helps.
Send me a message for Action Replay Codes.

So far, I know of only 3 ways. 1. Get Mystery Gift and than use WiFi connection during the event time to recive the key item required to get the legendary Pokemon. 2. Trade with a friend who has one/all of those 3 Pokemon. I don't think she/he will do that though. 3. Sorry to say, but the third option is to use Action Replay. The fourth option is to hack the game but I highly recommend NOT to since it could damage/destroy your game very badly. These tip-like-things above are for all the Sinnoh Region games: Pokemon Diamond and Pearl, and Platinum. Hope I helped.

once youve been to canalyve city go to the pokemart and get the members card. take the members card to the inn and go to sleep, you will wake up on new moon island. thats how you get darkrai. to get arceus you need to go to all the pokemon centres in sinnoh and once youve been to the 1 in sunyshore city talk to a lady in the union room she will give you the azure flute but you have to have wi-fi, there are many more ways to get arceus. i dont know how 2 get shaymin

you have to trade for evey thing ect for arcus because you have to go to toysrus to get it or ask a bubby to trade it to you

All the pokemon you seek are found in events... Which ALL of them ended... You need Wi-fi connection first for Mystery Gift. Most of the events BUT Arceus are from WFC. Here's the rundown: Darkrai- need Member's Card, Shaymin- Oak's Letter. Arceus was a special event at Toys 'R' Us. The event was within the store wirelessly and just ended yesterday.

ok I know how to get darkaria and shamin without action replay but I don,t know how to get arceus.here is how you get shamin first beat pokemon league and then save go to mestery gift and if you have wierless go to recive mestery gift from wifi.here is how you cath darkaria I dont know how to get the key but you walk in to the pokemart and there will be a grenn close guy he will give you a key and then go to canalave city go and then go into that house that has a door that won,t open walk in and this guy will lay you down in a bed and youl go to sleep.I hope this helps.

i think shaymin and darkrai need a nintendo event or a mystery gift arceus same thing but you need a azure flute go to mt.coronet use it right in the beginning where Dialga or Palkia is and a weird path appears to arceus