In diamond or pearl, it's possible to get Darkrai and Shaymin without cheats. However, Arceus really needs cheats. If you are using an early jap version of D/P, use the Surf glitch on Aaron's door(put it up on Bulbapedia or Youtube. It'll show some results and explanations,) or if you are using a later version of D/P and it's english, you should use the Tweaking glitch. (Again, use Bulbapedia and youtube as they have a complete explanation.) Those are the currently known ways to get Darkrai and Shaymin without cheating.
Sorry, but to answer this question truthfully: You don't. They're event Pokemon, so the only way to get them is to have gone to the event, get them from somone who had, use Action Replay or a similar device, or get them from someone who has done so. The surfing glitch at the Elite 4 doesn't work in the US version, so don't bother trying. Unless you already did. And either way, it can't get you Arceus.
Well its quiet easy actually i go them all level 100 each! well you can figure out Darkrai in sailport city (dang it i forget the name i lost my pokemon game cause of my stupid sister!) and i havent played it in sooo long! Arceus you need the flute and go to the mountain where you got palkia or dialga and it will play automatically and magical stairs will appear (not kidding). And for Shaymin use surf to get to the elite four then walk 500 to 600 steps to the right then 600 to 1200 steps straight down then you can hint use on your step counter! its easier than you think!:)
i know how to get a shaymin without any cheats in pokemon diamond just go to the end of route 224 when you are at the end just go straight you will see a long road ahead called seabreak path still keep in going straight till you reach the flower paradise in the middle of it you will see a cute little shaymin. this works i got my own shaymon like this
on platinum ive done shaymin....
:u need 2 go up roughly north east cant remember what the town is.last 1 u get 2 i think.u will meet proffessor Rowan by a giant rock and u speak with him.u hav 2 do something 2 get the special rock 2 activate.shaymin will appear and flee north up a very long road.u cycle up the road and u would eventually get at the end where there is a lot of flowrers.shaymins standing there and u battle shaymin.shaymin is pretty hard 2 catch so u hav 2 hav a master-ball 2 catch it.i ran out of pokeballs!:D
Darkrai:u go west to the far town.cant remember what its called.sorry.again.and u hav 2 of just about comppleted the game 2 get 2 darkrai.u go on the same boat as u did 2 get 2 the other island.u should turn up a similer island but much more spooky.u battle darkrai and it is very hard 2 beet so u hav 2 hav another mester-ball again 2 catch it otherwise its pretty much impossible!:D
thats all i no.
You get shaymin with oaks letter and go to rout 227 and shaymin apprs and you battle him/her.
You get arces with asures flute and go to sperr pillar and go up the stairs and battle arces.
You get darkrai with the member card and take the boat where the 6th gym leatter is and take the boat to new moon island
arceus azure flute never worked for me but i got 17 arceus, darkrai i cant rember but you go to this area everyone is sick you go to bed wake up thengo to this area and you get put in darkrais area, and shaymin i never gotit