I just discovered that the Mac Mighty-Mouse can be configured to do the middle click in JOSM. JOSM needs the middle click to show way numbers. I can't find any other way to discover a way number in JOSM, but it is free software so I won't complain.

In the Mac OSX System Preferences keyboard & mouse panel, the Mighty Mouse scroll-ball can be changed to "Button 3". For me, this worked to get JOSM to display the middle-click info box.