Since you got the cable from Valley, it's almost certainly an FTDI chipset. The Prolific driver is completely useless for this cable. As I've learned from others on this group (Thanks Robert Terzi):
The RT Systems cables I've seen are made with FTDI chips. However they install a custom USB Vendor ID and Product ID (VID & PID) on them, so they aren't recognized by the generic FTDI virtual com port drivers.
I've got a Mac and one RT systems cable (FT-60) and one Valley cable (FT-8800). Your most direct path to a working solution is to get the generic FTDI driver from http://www.ftdichip.com/Drivers/VCP.htm
It's possible to edit the Info.plist file in either the RT systems driver to work with a Valley cable, or in the FTDI driver to work with an RT systems cable, which I've done with my FT-60 cable, so I don't need both drivers installed (and I have a third FTDI cable unrelated to radios)). But you have to know what you're doing with plists and vid/pids, and have the motivation/need to do it.
The FAQ page that David referenced doesn't mention the generic FTDI driver, and IMHO it should.
-dan
Message: 3 Date: Tue, 14 May 2013 01:07:15 -0700 (PDT) From: David LaBier davidjlabier@yahoo.com Subject: Re: [chirp_users] new user, unable to connect to FT-7800 on a MAC To: Discussion of CHIRP chirp_users@intrepid.danplanet.com Message-ID: 1368518835.79248.YahooMailNeo@web122301.mail.ne1.yahoo.com Content-Type: text/plain; charset="iso-8859-1"
What driver are you using for the cloning cable? Read below. What radio are you using? http://chirp.danplanet.com/projects/chirp/wiki/MacOS_Tips