I'm not sure that libspnav is an option for OSX - at least, I think it never used to be, since the homepage says it supports only serial spacenavs on OSX - and I guess most people (like me!) use the USB ones now:
But having said this, I found this post which suggest someone at least was looking at OSX usb compatibility, I have no idea if this patch was merged:
There is some possibly out of date info on the FreeCAD side here:
https://www.freecadweb.org/wiki/3Dconne ... ut_devices
I find it's very nice: once you have trained your left hand to use it, it's like holding the object in your left hand. It takes a couple of hours to train your muscles, bit like riding a bicycle or something, but then it becomes quite instinctive.