There are two registrations, which is what is confusing.  The first one is for the HealthCentral site and then you have to register again in order to post to the forums.  When the forum page sends you to a log-in screen, skip logging in at the top of the page and scroll down past a menu of pages until you see another log-in section.  That's the log-in fof the forum, but you need to click the "register" link first since it is separate from the HealthCentral log-in.  Then, whenever you see that log-in page when you're trying to use the forums, scroll down to that second log-in section and use the e-mail address and password you used when registering for the second time.