When you subscribe to a forum it doesn't send you email notification of any and all posts. It will only do that for you if you post a thread or a reply within that forum and request email notification. When you click on the "Subscribe to this forum" button you'll then get a "Your subscription for that forum has now been added" message. This means it has been added to your user cp. If you go to your user cp you'll find that forum listed there under "Subscribed Forums" and if there have been any new posts to it there will be a yellow indicator on it just as there is on the main forums list for forums posted to since your last visit.