Here's how:
Go to the forum. You probably have it bookmarked, but just in case, it's at
http://www.radiantrecovery.com/cgi-bin/bbs-new/webbbs_config.pl

Scroll down to the very bottom.
Just under the Post New Message box, there are a set of links. The first one says "Post New Message", the second one says "Subscribe/Unsubscribe".

Click on the Subscribe/Unsubscribe link

Put your email address in the box provided, and click Send Address.

Hope that helps,
Kitty