Alternatively, if you want an email for each new posting, you can subscribe to a forum category. There will be a listing of the forum hierarchy under the main title and to the right of that will be a blue subscribe link (that changes to unsubscribe if you are already subscribed).
The catch is that there has to be at least one posting in the category in order to subscribe to it. Another catch is that I couldn’t find a way to subscribe in that manner to all of the forums, so earlier today I added the RSS feed for all forum postings. As you have likely discovered, the “RSS Site Feed” does not include forum postings. 🙁