I have activated the plugin and have created a Category and Forum, with a 1 Topic inside (no sub-forum). When I click on the forum, I get my site 404 page. I also have a completely empty Forum, and it also just shows the 404 page when I try to navigate to it.
Try it yourself. The forum is at http://scotpcg.org/forums/
Is it a permissions thing? What is shown if the user has no access? I have granted guest access to everything as far as I can tell.