I don't think that JForum 3 is currently (or for the foreseeable future) in a state where it can be run properly. I advise to check out the JForum 2 fork at https://code.google.com/p/jforum2/; that's where JForum development happens at the moment, based on a stable version.
Hey, check out my mega multi devastator cannon. It's wicked. It makes this tiny ad look weak: