I don't have a Mac but after seeing this question asked before, I believe you don't have a USB driver installed for your programming cable. If you have one of the cheap programming cables with a clone/counterfeit Prolific chip in it, then you need a special driver. I believe that once you have a correctly installed USB driver installed, there will be a USB selection along with the Bluetooth selections.
Check the CHIRP Documentation page
http://chirp.danplanet.com/projects/chirp/wiki/Documentation Especially the MacOS_Tips page