We have now (hopefully!) fixed the signature settings box on everyone's profile pages so you can go in and add a new signature. When you're logged in, click on your name at the top of the page, go to Settings, scroll about half way down the page and you'll see the Signature Settings box. Type your sig in here and save changes. It should then take a few minutes to pull through to any new posts you make.
NB: In Drupal 7 (the new platform we're on) the signature settings box is limited to a maximum of 255 characters, so if you type more than that the page won't save.