When you first enter Nintendo wi-fi through internet connection on your ds it'll give you a code on your ds. Write down the code and go to pokemon-gl.com and sign up, using your code to register your black/white game. Once you sign up every 24-hours you should be able to login to your pokemon dreamworld online at pokemon-gl.com and even get up to 50 pokemon total (though i've heard the limit will be removed soon, and that it's already gone in Japan) with special abilities and occationally moves you can't get in-game.
You can even custumize the C-gear you get in the game, which you get by putting in passwords in the promotions page on your account on pokemon-gl.com.
BTW if you want a password check out other forumns with the passwords.