There is a building on Route 15, and it's like Pal Park since you can use it to transfer stuff from past games. You can only use this after you got the National Pokédex in Black or White though. Everything will be explained when you go inside the building.
You need two DSs though (one with Black/White and the other with a past game). You pick six Pokémon to send over, and they have to follow the restrictions: they can't have an item, can't know any HMs, can't send over the Spiky-eared Pichu, and you can't send an egg. You also cannot send Pokémon from Black or White back to the older games, unfortunately.
oh and you need to beat the game first if the whole national pokedex thing confused you