In reply to hayden_sterckx:
You don't. You just start up forge, then launch Minecraft as if it were the new one. It should still work.
In reply to king_creeperz:
Actually, I tested it, and it works fine.
PS : I edited the message. I meant old launcher. Sorry! :)
Buuuut... As far as I know, the old launcher doesn't work properly anymore.
Me neither i'm getting exactly the same problem.