To use a signature you need to click Profile at the top of the page.
A subsection appears below this. Click Signature.
Here you may edit your signature.
Signature: Will appear under all of your posts.
Click Preferences. Scroll down to the bottom.
Be sure you say 'Yes' to 'Always attach my signature'.
Your signature will now always show up after your posts.
+Rep please! HOW TO REP TUT