http://wiki.xoops.org/wakka.php?wakka=CannotLogin
http://wiki.xoops.org/wakka.php?wakka=CannotPost
XMDT FAQ - Explanation <--- try this one.
Also a search of the forum would help a lot, there is tons of info, just try search queries like 'login problems' 'can't register' etc, etc.
Hope this helps