The answer was in the first reply by Crazy_Rob: http://forums.digitalpoint.com/faq.php You need to add it in your Profile and it will appear after you've made the minimum number of posts (10 I think).
Our system has automatic signature guideline enforcement. Currently the restrictions are: Up to 250 characters No more than 3 lines No more than 4 links Images are not allowed Signatures and live links within posts are not available to users until they have at least 10 posts.
When your account is "upgraded" go to http://forums.digitalpoint.com/usercp.php to edit your signature.