The Critique Circle is open to everyone. It only closed to non-500 members briefly in July last year (for about three weeks).
You would need to be showing at least one critique credit in the left hand corner to be able to do it. So if you have posted more articles there than you've actually critiqued, then that would explain why you couldn't post.
However, if you have at least one critiquing credit showing, then I'm not sure what the problem is.
If you have enough credits showing (or should have), it would be best to contact FaithWriters direct via the Priority Support form here:
http://www.faithwriters.com/myaccount/m ... upport.php
You'll need to login to your FW account to send it.