Select the Database Window. Go to Tools|Startup on the menu. In the Startup dialog, select your menu bar in the Menu Bar box.
You will still be able to switch between the default menu bar and your custom one by pressing Ctrl-F11 on the keyboard. When you are ready to distribute your database and don't want the user to be able to access the default menu bar, go to the Startup dialog and uncheck "Allow Access Special Keys". Ctrl-11 won't work any more, and neither will F11, Ctrl-Break, or Ctrl-G.
If you need to switch between your menu bar and the default one in code, you can do it by setting the Application.MenuBar property. Assign your menu bar's name to get your menu bar, or assign an empty string to it to get the default. (Don't assign "Menu Bar" to it, even though that's the name of the default menu bar. It confuses Access when you do this.) Rick Sprague