Unless you have an Action Replay or other cheating device, you'll need to have a special item only given away at Nintendo events, like at Toys 'R Us. Sometimes Nintendo gives them away, but you can't get Darkrai in the game without a Member's Pass.
By Pokemon rangers or cheating
none
yanma
It's already out !
Marry a rich wife
No Pokemon evolves into darkrai.
You have to buy Pokemon Platinum. (Its a separate game!)
yes there is some new Pokemon on plantinum. like girantina
You can only get one
in the snowy mountains but you have to eveolve it
turn back cave.
The 4 powerful guys that you battle at the Pokemon league.