Route 15, at the top part.
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.