Ultimately, this seems to be a presentation shortcoming of the Java Runtime Environment (JRE) on the Mac.
On Windows, the Zoom in/Zoom out keyboard short-cuts are mapped to NumPad+ and NumPad-, respectively. On the Mac, however, the 'NumPad' detail is somehow lost and instead NumPad+ is presented in the Menu as a simple "+", NumPad- as a simple "-".
Using NumPad+ or NumPad- on the Mac keyboard in combination with the Cmd key works as expected!