Go to http://forums.digitalpoint.com/profile.php?do=editsignature and use editor. My edit options http://forums.digitalpoint.com/profile.php?do=editoptions is set up to use standard editor. You may have to wait for CRON job to run to set your eligibility.
You can't use a signature until you've been a member for a full 7 days, at the time of posting this thread you had not been a member for a full 7 days... That's the problem. You should be able to use your signature fine now.