Right click on the siggy, go to "Properties" at the bottom, and highlight the Adress/URL. Then right click the highlighted address and click copy. Next go to the top of the screen, where there is a blue bar with "User CP" on it. Click on User CP and then on the side there will be a "Edit signature" button. Click it and it will be like a post box. At the top there will be a small yellow button with two mountains on it. Click on that and a box will come up. In the white space, right click and paste the URL in it. Then click OK and the URL will be in the post box. Then click "Save Signature".

Hope that made sense!