This forum uses several layers of anti-spam filtration. One of these layers refuses new users the ability to post URLs in either signatures or the body of their posts. After a reasonable number of non-spam posts are made, the filter automatically disables itself.
To solve the issue, either make a few sincere contributions to some active threads or post your URLs in a way that the system won't delete them, but that a reader can still make enough sense of them to type them into their own browser, i.e.: somedomain dotcom/mypage dot html.
A separate layer of antispam filtration comes in the form of automatic akismet post/thread filtration. if the system decides that you have enough spam keywords or links to commonly used spam sites, your post may be dumped by the system into a moderation queue.