You go to a place called Poke Transfer,where you can trade Pokemon through generations after you beat the game.
Like the games in Generation IV, Pokémon Black & White have a feature which allows you to transfer Pokémon over from games of the previous generation. This is done in Black & White through the PokéTransfer feature. Located within Route 15, there is a building similar to Pal Park of Sinnoh & Kanto where you can connect to past games for the Pokémon after you have obtained the National Pokédex. When you enter the PokéTransfer Lab, you'll get an explanation. You can then access the transferring program from this area whenever you want
To do this, you will require 2 DSs, one with Black or White, and the other with Diamond, Pearl, Platinum, HeartGold or SoulSilver. Then, when you go to connect, you will get a DS Download play application which will link in with your fourth generation game. Like Pal Park, you send over 6 Pokémon at a time from your boxes. Unfortunately, however, you cannot transfer Pokémon with hold items or Pokémon that know Hidden Machine moves. Here are all the restrictions
No Hold Items - (Game automatically puts them back in your bag)
The Pokémon cannot know Hidden Moves (Cut, Fly, Surf, Strength, Defog, Whirlpool, Waterfall, Rock Climb) - Do note that you can send Pokémon with Defog through HeartGold & SoulSilver and Pokémon with Whirlpool through Diamond, Pearl & Platinum
You cannot send Eggs
You cannot transfer the Spiky-eared Pichu
Do note; If you transfer the Raikou, Entei, Suicune or Celebi obtained from events through the PokéTransfer, they will not unlock the Zorua and Zoroark events. To do that, you must use the Relocator
Mini-Game & Limits
Unlike Pal Park though, the method of obtaining them here is a simple minigame. On the top screen, you will see various bushes and the six Pokémon will jump between various ones. Your task here is to use a Bow which shoots out Pokéballs to try and capture them. This is controlled with the touch screen and requires good aiming. There is a reticule on the topscreen so you can see where the ball is likely to land. There is also a time limit to the capture here. The rarer Pokémon tend to move faster.
As time progresses, a pink cloud will appear which will make all Pokémon fall asleep for a short time so you can capture them. The Pokémon you fail to capture during this minigame will return back to the game you're transferring from and can be transferred again.
Also, unlike Pal Park, you can transfer Pokémon at any time with no limits. As soon as you have transferred your six Pokémon and completed the mini-game, you will be asked if you wish to transfer more. This can be done straight away and any number of times so you can get your Pokémon across easily. As you fill up your boxes, your box limit will increase until it hits 24 boxes.-Serebii