I know how to set Super_L (WinKey) button to open the menu.
gconftool-2 --set /apps/metacity/global_keybindings/panel_main_menu --type string "Super_L"
Currently, in order to close that menu, I am having to mouse click outside the menu area. I need to use the Super_L as a toggle button. (pressing one time would open and pressing second time would close)
So what I need now is to be able to close it when pressing it second time.
Unfortunanly, Gnome doesn't allow that. They could have done a toggle to make it work.
Since you already pressed a button to open the menu, your logical next step is to use the arrow keys to select an item and press enter. If you activated the menu for other, possibly illogical reasons, just press the give-up key which is unfortunately mislabeled as Esc.