Are you using GNOME/KDE?
I dont think that would matter much though! There should be a button for both and I guess its titled Log Off!

(not sure as I'm not using linux now)!
Once you do that, you'll be taken back to the login prompt box. AFAIK, the option to shutdown, restart... will be there!
What is in the menu when you click on the button(the one on the tray)