I just did mine and it went smoothly for me. Have you gotten to the stage of generating code? If so, select the BB format and click on generate code. When it completes, there will be a string of text in another box (starts with img). Highlight all of that code and Copy it. Then go to your profile and delete everything in the box that says Signature and click on that box and Paste into the box. That should do it.
If you're still having trouble, try to let us know what step you're having trouble with. Does it not generate the code? If so, is there an error message with it?