Do you have, immediately to the right of the shutdown button (in the start menu), an arrow which you can expand to find the dialog tree you're looking for? You can also access the "log off" button by pressing ctrl-alt-del.'
If you want it to appear on the start menu without clicking the arrow, you can adjust the local machine policy by typing in the search/run field, "gpedit.msc" and pressing enter. Then navigate to "User Configuration", "Administrative Templates", "Start Menu and Taskbar". Modify the policy "Add Logoff to the Start Menu." Change it to enabled. Reboot.
Does that help ?