Check what it is assigned to in the keyboard shortcut editor (under "Edit" menu).
To see your current set and quickly locate under which of the dozen-and-some categories "zoom" is hiding, click the Show Set button and use the local "find" command of your OS plain text editor.
"Cmd + +" would indicate that you need to press command, then the "+" button -- the regular way of expressing that would be Cmd +Shf + =. Perhaps you *need* to use the numerical '+' on your system? (It's not required on my Mac, I can use both the '=' and numerical '+'.)