How do I drive traffic to my non-profit website and get people to post on the forum?

What type of forum is it? Maybe that will help us give you ideas. Also, maybe if there are similar forums you can frequent those and try to lure people from them over to yours. That's kind of shady but people have done that before.

yeah it's hard. Posting REAL posts on other people's blogs in the same space. joining blog hops, participating in other forums.

Post things that are interesting to them in an effort that they'll participate. Comment on other blogs. Write guest posts. Tweet up a storm. Engage them.

I really like forums like this one that lets you post anonymously - it's a great way to lower the barrier.