If you know someone with Pokemon Black and they are playing their game, go to the Entralink via the C-Gear (make sure Wireless is on if you're playing a 3DS). Enter the portal at the bottom of the area and head into the other players game. Go to the Black City, unlocked or not, and talk to some of the people there. With any luck, they'l turn up in your White Forest the next day.
In other words, you just someone you can steal residents from... :)
Hope this helps...
My answer here.