Once you've been a registered member for 20 days and have at least 20 posts, you'll have your signature enabled. Then you can add your ticker in your User CP (in the purple bar near the top of the page). The restriction was put in place to discourage spammers, who would join and immediately post a lot, put in a spam link in the signature, and give all of your moderators BIG HEADACHES trying to clean it up.