sorry, another quirk of the forums. two methods:
1.in your profile, under the second tab preferences, about 2/3 way down, you need to select yes for "always attach my signature". hit save. from there on, it will show your signature (but not in old posts).
2. when writing a post, under the message input box, there is an options bar. expand it, and there you can manually check "attach my signature". this is for that post only. and does not apply to past or future posts. youd need to do this every time....
hope this helps.
cheers!