With many menus, the ACL to use is menu_active() as in
eval menu_active("File.Open...", 0)
eval menu_active("File.Open...", 1)
However, when Arbortext first opens, it is in text mode. The Entities menu is not available for a text file since there would be no way to add an entity to a text file. If you want the Entities menu, you must first open an XML or SGML file.