I believe that should be Command =/- for zoom in and out.
Works here on El Capitan 10.11.3 using -/=
That's really strange. I changed my keyboard layout to from German to US but still not working.
I can only zoom in and out in the GRID View but not in the Full Frame View (as Scott Kelby described in his book).
In the Grid view I have to use the keys CMD and # to zoom in and CMD and - to zoom out. But in the Full Frame View I didn't find any working shortcut.
It's getting even stranger. There are more shortcuts that do not work. CMD+ALT+0 should hide the grid - nothing happens.
I wonder if it is my Mac or Lightroom!?
I am not able to get the shortcut for Cmd+Alt+0
As far as I am aware, Alt key is not there on Mac keyboard. It should be having Option key.
And if you will go to Library module > help menu > Library Shortcuts > it lists the zoom in shortcut as Command + = keys.
Test if Command and = keys works for you for zooming in.