How to make Menu.add_command() work in tkinter on the Mac?
I don't think you can do that with the native ("Aqua") Tk on OS X and you probably shouldn't try. OS X native menus don't work like that and Tk tries to follow Apple's Human Interface Guide for menus. You need to have a menu bar with dropdown cascades.
The TkDocs website has a good introduction to Tk menus and their platform differences. (You could use an X11-based Tk on OS X, but that is not recommended as Apple does not ship X11 servers anymore with OS X and your app would look and behave oddly for OS X users.)