(Hope it's okay to post this link here! If not appropriate, then I trust our moderators will remove it.)
If you have been active long enough to be allowed a signature, but you are still having trouble finding the page that will allow you to add one, you can access that functionality via this link:
As with any sort of forum post, the signature line supports smileys, hyperlinks, off-site images and special formatting.
There is a limit on the acceptable character length for a signature, but don't worry about limits unless the page tells you that your signature line is too long. If you do hit the limit, simply edit it down and try again.
- M.