Getting a forum off the ground more than nearly anything else is one of the most time consuming things for a content site publisher to do. In some cases it takes years to get enough posts for the forum to truly become active.

The issue is that if people have no threads to reply to then they are less likely to register, and if no one ever registers then there will be no threads to reply to. It is very much a catch-22 situation. You will get the occasional person who registered to ask a question, and after a significant amount of time you may have enough of those people to get the ball rolling. However in general forums need a critical mass of posts before they start getting active with daily repeat visitors. I estimate that mass to be at least 1000 posts.

Read more about setting up a forum with a little help from Adsense at http://www.sitepoint.com.