If the hover-over isn’t working, try just clicking on the topic. It should take you to a page that filters all questions by the topic—with an option for you to “Add to Profile” (or “remove” if it’s already on your profile).
You can also reach the topic page via search. Search for the topic. Scroll past the usernames and the Google ads. Somewhere in the main results you should see “Fluther: [your topic].” That will take you to the same topic page, with the same “Add to Profile” option. (At least, that’s how the search works on my computer; others say just use a search engine… yep, searching “fluther: [your topic]” will bring up the topic page in the search engine. That’s probably easier.)
Additionally, on the right side bar on the homepage (and some other pages), there’s a “Popular topics” below the Community Feed. You can again hover-over or click the topic buttons there.