"Lock screen" doesn't work

[lewst yahoo com]

I'm running RedHat 6.0 with gnome.  I need the
ability to lock my screen when I leave for the
day.  When I bring up the main menu, and click
on "Lock screen" (about 4 choices up), nothing 
happens.  It seems as if nothing is even being
executed.

What program does "Lock screen" call when clicked?
I suppose there is the possibility I don't have 
that rpm installed.  Barring that, does anyone 
else have this problem?  Thanks.

(Please CC: me on all followups)

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