(First of all, let me clarify that I acknowledge and prefer a 3 button mouse as a better choice for Rhino than a Magic Mouse. However, sometimes the Magic Mouse is what is available on the desktop.)
Using a MacBook Air, sometimes (manytimes) I can’t Zoom in the Rhino viewport (GH works fine). I have found an easy (but annoying) solution: Scroll once with the trackpad (which works always), and immediately Rhino recognizes the MagicMouse scroll event.
This happens in Rhino7, Rhino8 and I also remember it happening in previous Rhinos and MacBooks.