Gah! I made all my ranks correct in the admin options and still can't get it to work! I even checked forum options!
Your problem is just that you didn't purchase the forum portion of the site. If you head to Admin, you'll see the following near the top:
Until the forum package is purchased, you won't have the option to post any new threads.
The reason forums aren't included as a part of the base system is since forums tend to be the largest, most visited part of a site, if a customer is using their own forums hosted elsewhere, we don't want to charge them for that service (for lightening the load, so to speak). So the base system is then cheaper than it otherwise would be if it included the forums.
Make sense?