There's a magic empty click spot somewhere on the Topic View pages. View any topic in the forum and the link is in there. It's hidden, so you'll need to mouse around the topic pages as you're getting up to date on your unread posts. Actually, since you're reading a topic page right now, it's on this page as well!
When you find the spot and click on it, you'll be sent to a page that will give you the rest of the instructions.
I think it's all pretty self explanatory, but if you have any questions or comments, please let me know.