As of version 2.2, Custom Menusets on Mac OS are supported. To use them you need to turn them on in your app settings.
Go to Deployment Profiles for your app
Click on Edit in the Mac Deployment Profile for your app. If you haven't already created a Mac deployment profile, you can do this by following the steps in this lesson.
On the next screen, click on the Mac icon.
Check the box to Enable Menubars. You can now use Custom Menusets in your LiveCode for FM app.