Right now, webtv users can log in by clicking on the "Login to post a new forum topic" on this page:
This should also be the link that all save to view the forum. This link opens directly to the main board of the forum.
You can also get to your account preferences then by clicking on the Login link at the top of any page, and then click on your name from the "Who's Online" list. (that list should now be displayed on the login page)