You have to enter it in as HTML onto each message, and then click the "Check here if HTML tags are in the message (not including signature)." box.
I have used 1st Page 2000 (free off the Internet) to format the HTML, copy it into a text file, and then keep the text file open when I'm on the forums.
Other times I don't bother with the sig.
If you're reasonably proficient at HTML, you can do it on the fly, and do a different one each time.
We actually *did* have the old sig box under Preferences, for one day, but then it was gone again. Now we only have access to HTML.