If you look just above the "Powered by Zeroforum" at the bottom of the page within a thread, there's a "Rate this Thread" dropdown. All you have to do is select your rating from that box. If I understand it properly, the rating is the average of all the individual ratings.
At the bottom of each thread (visible only to members who are logged in) is a selection box titled "Rate this thread" Each member can give a rating to a thread only once per thread, just like a poll. lurkers don't rate, got to be logged in. Values can be from 1 to 5 stars. It seems like all of the members ratings are averaged to generate the rating that is displayed. The ratings are only displayed int he forum list, (not in the "active topics" or other search results listings.