how can you get to the dream world?
The Pokémon Dream World (Japanese: ポケモンドリームワールド Pokémon Dream World) is a special feature of Pokémon Black and White and Pokémon Black and White 2. Operated via the Pokémon Global Link website, the Dream World allows players to send a Pokémon to the Internet to obtain items and meet other Pokémon, making the website like a Generation V analog to the Pokéwalker.
Players may access the Dream World by creating an account at Pokémon.com for international players, at the Pokémon Daisuki Club website for Japanese players, or at the Pokémon Korean site for Korean players. To have full access to one's account, players must first send a Pokémon to the Dream World by using their C-Gear's only Online feature, Game Sync. After doing so, players will have full access to the Global Link site. An account may only have one Pokémon Black and one Pokémon White game with accecess to it.
Hope I helped :) Source
http://pokemondb.net/pokebase/101510/what-is-the-dream-world
and
http://pokemondb.net/pokebase/70439/what-is-the-dream-world-how-does-it-work-and-how-do-i-use-it
