David,
If you have customized an existing menu item, like File->Open, take a look at the help page for the menu_change ACL command, particularly the -active switch. If you are talking about a menu item that you created using the menu_add command, you'll notice it also has an -active switch.
You mention that it's a context menu, which means you'll need to use ":EditPopup.<path.to.menu_item>", like what is shown in one of the menu_change examples. EditPopup is for context menus on the main Editor document view windows (edit and docmap).
If neither of these commands help you, reply back and let us know how you create or change the menu item.
