To add a signature, click the User CP link in the top left corner under the logo for the site. There is an icon on the left side of it.
Once in the User Control Panel, there is a sidebar on the left side with three sections with black backgrounds.
The second section which reads "Your Profile" contains the "Change Signature" link and has a pencil on the left side of the link.
I hope this helps you and welcome to the forum.