To get your signature to always show up, just click the "Profile" link at the top.

Then click the "Preferences" link.

Then click "yes" on "Always attach my signature".

Then click "Save" to save your settings.

Or an graphical, minimum-confusion version of these instructions: http://img501.imageshack.us/img501/6195/screenshot51oe9.png

Of course, you also have to think up a signature, which can be done by clicking the "Signature" link next to the "Preferences" link. But if you already made a signature and feel puzzled by why it won't show up, these instructions are for you.