Follow this and you shouldn't go wrong.
How to post a sig!
First you need to host it at a host site (eg. Tinypic, etc)
Copy the link for your sig and return to the board.
Now click Profile at the top left of the page.
Now in profile goto and click on Edit Signature.
Once in there goto the big main box and put in
after your link.
So it looks like this
Then goto just below the big box and click on Save Changes
That now has you sig linked to the board.
All you need to remember now is to tick the Show my Signature box, below the message box everytime you make a post.