I can't say specifically for a MAC but the command you want to change the shortcut for is "Zoom In (scrollbar) - not just "Zoom in" - that is the one where you define the window area on screen.
It should be set to this by default.
Make sure you use the plain '+' key on the numeric keypad if you have one.
Don't use the '+' above the '=' key as you need to press SHIFT for that one.
That is if you MAC has a separate numeric keypad.
If it doesn't that is why you have to press 'SHIFT+' as main key is '='.
The '-' works as that is the main function of that key.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11