At the top right of the forum page there is a link that says "my profile". Click on that.

Then, click on "edit my profile" at the top of the next page. On the next screen, scroll down to the section titled "profile fields".

In the signature field, type in whatever you want, and then your web address. It will be made into a link if you use the following format: www.yourwebsite.com

Let me know if you need more help!
Kathy