Hey hello!
When you hit Reply, below the white text box where you type your response should be a blue box with a white plus-sign on it, right next to "Attachments and other options". Click the blue box, and a whole bunch of things should appear. Choose the signature you want from the drop-down menu there and presto!
Let us know if you still have problems after that!
Also, some forums don't show signatures, so that might be it too.
-- Wally.