In order to subscribe to a topic, go to that topic on the forum and click on the down arrow next to wrench icon . You will see a drop-down menu appear and in that menu will be a "Subscribe topic" link with a checked checkbox next to it . Now, I know this is not the most intuitive thing, but that means to click on the link to subscribe! I know it's not the nicest design in the world, but click that link and then it should change to a link that says "Unsubscribe topic" without the checkbox checked .
Again, it is not the best design and I am looking to upgrade the forum to the latest version. It may be more intuitive, but probably not.
To subscribe to a whole forum, go to the forum and scroll down to the bottom and find "Subscribe forum" and click on it . You will now be subscribed to the forum. Again, not the best wording or imagery.
To show what topics and forums you are subscribed to, you can go to ucp.php?i=ucp_main&mode=subscribed to check that you have subscribed.
I hope this helps!!!
Long Island, New York