List of Accounts Does Not Show on Top Menu Bar

The top menu bar all the way to the right of every screen shows icons for time machine, Bluetooth, wireless network, speaker volume, battery level, Day & time and then for Finer. It should show an icon for user accounts - to switch between different users accounts. It should, and it used todo so on my iMac under OSX 10.4 but its not there now.

This is to the far right from the Apple logo and in the very top right of every screen. Any idea of how to make the list of user accounts show up again?

Thanks everyone for the help and expertise!