Once you're logged in, find
Forum Actions near the top of the page. Click on it, then
Edit Profile.
Edit Signature will then be in your left column, under
My Chat Profile, contained in
My Settings.
One thing I learned: If you've changed something that required an email to be sent to you for confirmation, you'll need to check the email first before being able to change your signature. Otherwise, the option to change your signature is not displayed.