In order to make this work, you first need to make sure that your X server thinks you're using a Microsoft-like keyboard (one with the Windows key). Under XF86 v3.3.5 on RedHat 6, this is done by editing the file /etc/X11/XF86Config. The goal here is to convince your X server that you're using a keyboard that comes with the Windows key on it. Within this file look for a line that reads (or something very similar to this):

XkbModel "pc101"
and use your favorite text editor (you'll have to be root to do this) to change that line to read:
XkbModel "microsoft"
Save the changes to that file. To make sure the changes have taken effect, you should log out of your X session now, then log back in again. After you're logged back in, right click on the Gnome Main Menu icon (the big left foot icon).

From the menu that pops up, choose 'Panel' and then 'Global Preferences'. In the window that is opened, click on the 'Miscellaneous' tab. In the key bindings section, make sure that 'Enable panel key bindings' is checked. Next, press the button labeled 'Grab key...' that is to the right of the text field 'Popup menu key'.

Now press the Windows key on your keyboard. Doing this will bind the Windows key (called Meta_L by Gnome) to your Gnome menu. Now press the Windows key, and the main Gnome menu should appear wherever the mouse pointer is located. And there you have it!