When you are in a thread you like look right above where you posted your post. You see "thread tools". Click on that, then click on the last one that says "subscribe to this thread". On the next page click on "add this thread" or something like that. Click on it. Your done.
To find that thread go to "My Rollitup" (top right of page).
Hope that helps.
On the second page it actually says "add subscription" not "add this thread".