This brings up the menu at the end of the current interpreter cycle. The next cycle does not start until the user has selected an item from the menu, or pressed ESC to exit the menu.
Flag 14 (or menu_enabled in the template game) must be set for this command to work.
ESC is usually the key that activates the menu, but this is not built in to the interpreter so do do this you need assign the ESC key to a controller using set.key, and then test for that controller to activate the menu (this is already done in the template game).
Setting Up Menus and Keys