It won't happen overnight, start posting current press releases, and it will become more searchable and that will help users find it, and hopefully they sign up, one thing I did was change my phpbb forum and make it where you could not view the forum without signing up, it has really helped with signups
Initially pay them to sign up and post. Offer free services, answer their questions. And use bots to make it look your forum is populated
A common rule is that you need to post yourself on different accounts for at least six months. Other than that you can pay forum posters/leech other forums.
Make sure that your forum has some unique spin in the niche, you don't want to be another make money forum, or another "health" forum.. make sure what you are doing is fresh and new. Also keep in mind that starting a forum is the hardest point. Make sure that YOU as an owner are posting every single day. Hold Posting contests to get the amount of content up and promote it through out forums (using Sig tags), writing articles and telling everyone you know about it.
Ok thanks, yeah i have notice starting a forum is not that easy lol but thanks everyone for the advice.