[chirp_users] MacBook Air will not talk to UV-5RPlus

I have a MacBook Air running OS 10.9.2. I just installed CHIRP 0.4.0 and Python runtime and tried to program my UV-5RPlus. I get an error message saying that the radio did not respond. I checked the cable (Prolific chip) and found that the computer recognized it. I should note that I have used this same cable to program the same radio on a Windows 7 desktop PC, so I know the cable connects properly to the radio. Everything works, except I can't download data from my radio. Any thoughts or solutions?

Bob DeFranco