Hi there, I made my guestbook also in the forum, because i like it very much. But when you want to post something, you need to be a member. I tried to put it on: everybody can post a topic (not only members) but I guess it doesn't work.. Can you have a look at: http://www20.brinkster.com/boffers/forum/
When you know the answer, can you please mail me or reply to this topic? ajaxside@yahoo.com
The forum is based around a registration system. Meaning users have to be a member of the forum in order to post. It hasn't been done before because it would take a lot of code modifications to the forum to make non-members post. And it's not something that's gonna be implemented any time soon into the forum.
But I have created an Anonymous Access mod for the Alpha version (the non-productive version). You can see it here: http://www.dsilvera.com/beta/32_a6/ I haven't released it yet. But even that mod won't fit your needs as it allows any user to post to any topic without registering. Won't be able to make it only for one category or forum.
But what you could do is register a guest account or something and let the users use that account to post. I did that here: http://www.dsilvera.com/beta/31_sr4/
But if none of the above works for you, then the answer is, no, it can't be done.