Due to a high level of dubious sign-ups, we've currently switched off the ability to register yourself as a site user. We'll probably switch it back on fairly soon, but in the meantime you can use the General Feedback category in the Contact form to ask to be set up.
You don't need to be registered to post a reply to a topic in the discussion area, or add a comment elsewhere, although we'll need to approve it before it appears for others to see.