I have asked for this before and always gotten an explanation why it does not make sense, but here goes again:
It is a standard convention on the Mac for <CTRL-+> to zoom in and to zoom out.
As I have mentioned before, Rhino repeatedly stops responding to trackpad commands on my macbook and then refuses to respond for extended periods of time. I can scroll with the keyboard but I cannot zoom. If I could zoom, some of the misery would be alleviated.