Click on the Profile tab that you see on the top which is next to My Topics,
Next Click on Edit button on the top right, next click on Profile information, which is the middle button. You will see the Signature box right at the bottom of the page, type in your signature in that box.
Remember, if you wish to add links to your other sites as your signature, you are allowed only ONE link.
"I am free of all prejudice. I hate everyone equally." - W. C. Fields :)