Okay, you will need:
- Two DS systems (it can be DS, DS Lite, DSi, 3DS, whatever)
- Wireless communications launched on each DS.
The recipient system is the system that has your copy of Black and White, and it will get the Pokemon, and the other system (xD) is the system that has your copy of Diamond, Pearl, Platinum, HeartGold and SoulSilver, that will give the Pokemon to your Black and White game. (Just to clear things up when I'm explaining xD)
- On the recipient system, head to the Pokemon Transfer Lab on Route 15.
- On the recipient system, go right up to the top and talk to the guy, and he will ask you to save your game and stuff
- Remember to turn on your wireless if you are on a 3DS (or DSi, if it has that wireless switch on the side)
- The recipient system will ask you to turn on your Nintendo DS Download Play on your other system. Do that, with Diamond, Pearl, Platinum, HeartGold or SoulSilver inserted into the other system.
- The games should find each other, and then just do what the games need to do on either of the systems.
Remember you can't trade items, and you cannot trade Pokemon back. Migration means you cannot trade back Pokemon.
Hope I helped. :)
Source: I just tested it on my game. xD