Try framing the object first and then scroll to zoom in and out. I've noticed that sometimes you have to frame an object before zoom will work.

You might also try looking in the shortcuts menu to see if you can find zoom listed there and what the correct key combination is. You can get to the edit shortcuts menu in preferences under the interface tab.

I don't know what the various differences are between a pc and mac tho, so I can't really be of much help beyond that.