In Black, you can't get to it. White Forest, not Black Forest, is only accessible through Pokemon White. In Black, where White Forest would be, there is Black City. So if you wanna get to White Forest, buy Pokemon White.
Ok, well first of all, you need the C-gear. Once you have the C-gear, tap "wireless" once you do that, click on "Entralink". It will warp you to there. When you get there, go straight up, and you...
You must have a friend with Pokémon white. Turn c-gear on and use wireless, entralink on you Black, then just click where your black city would be and you will teleport to their white forrest. Then...
you cant, you will find black city if you got black, and white forest in you got white you will find either places right from nimbasa city after you have defeated the elite four and N.