So to clarify what's happening right now:
On these forums, the yellow expandy box at the top right does not properly work. The issue lies in the cartalk.com handling of logins. As you have probably experienced, logging in from the forums via that yellow box takes you to your cartalk.com page and not back here as you would expect. There is not much the forums can do about that.
However! Logging in on cartalk.com and then clicking the Community link should work fine. I tested it extensively yesterday and today. If you're having trouble, I highly recommend clearing out your cookies and cache and trying again. It is possible that there are some shenanigans going on in your browser's cache. Give it a shot.
cartalk.com are working on the issue with their login handler, so that should resolve soon.