Bascially you have to be patient. When forums first open if they aren't attached to a popular website they usually take ages to grow. But there are a few things you can do.
Encourage your existing members to post as much as possible and also post lots your self. Make sure any new members are welcomed by people. Also encourage your members to advertise the forum in all the forums they post in and do the same yourself. Try and think of a reward you could give your members for doing this maybe a free graphic or something - in my forum we have forum money which people get doing these kind of things.
Also try and think of as many interesting topics as possible and post them to encourage discussion. People are more likely to join forums which look active than ones where there's lots of empty boards.
I hope this helps
