You don't.
You trade from Platinum, Pearl or Diamond.
You cannot catch Darkrai in Pokemon Black, you have to transfer it from Diamond, Pearl, Platinum, Heartgold, or Soulsilver.
Darkrai is an event Pokemon, and cannot be found through normal game play. Darkrai thus cannot be found in SoulSilver. Darkrai was an event Pokemon that was available for Diamond and Pearl, but not for SoulSilver.
Darkrai cannot be caught or encountered in HeartGold or SoulSilver Versions.
There is not a known Darkrai Action Replay Code for Pokemon HeartGold and SoulSilver yet.
you cant put you can in platinum version
You can't catch bastiodon in Pokemon soulsilver.
You dont catch Darkrai in Pokemon Pearl. You can catch it in Pokemon Platinum at level 40.
you catch him
You can't get Darkrai in soulsilver.
You get Darkrai by trading in SoulSilver.
Darkrai has the catch rate of maybe the lake pokemon( Azelf, Uxie, and Mesprit) seperate.
You can't catch Darkrai in Pokemon FireRed, it is a 4th generation Pokemon so therefore unavailable in any of the GBA Pokemon games.