Hello,
what did you try to do? There is no need to "add a topic" in backend.
If you view the page on the web, there should be a link "add new entry" or similar. That is where visitors may leave a comment.
If there is no link displayed, try guestbook 2.8.5 from here
http://www.websitebaker2.org/forum/index.php/topic,7920.msg75977.html#msg75977as 2.8.4 seems to have a bug (settings get lost).
thorn.