Quit OneNote before install add-ins.
You must quit the add-ins also if you want to update the add-ins,
Download Gem Menu for Mac OneNote
Download Gem Menu for Mac OneNote from product web page:
After download Gem Menu for Mac OneNote, Mac will pop up a security window refuse install the app.
Open "System Preferences" -> "Security & Privacy" -> "General" tab -> "Allow apps downloaded from:" -> "MacONGemMenu-188.8.131.52.pkg" was blocked from opening because it is not from an identified developer. -> click "Open Anyway" button -> to install.
After complete install, the Gem icon appear on Mac menu bar.
Where is Gem Menu installed?
Gem Menu install in "Application" / "Utilities".