Thanks Dan - A quick follow-up:

You probably need to install the driver. See http://chirp.danplanet.com/projects/chirp/wiki/MacOS_Tips

Done - thanks.

Use System Information -> USB to find your cable in the USB device tree. If it's a Prolific device, you can try the open source driver mentioned on that page...

What's the syntax I need to use in Chirp to point it to the cable? What you're looking at below is what System Information is reporting for the USB-to-Serial cable I'm using (Edgeport/1), plugged into a USB hub. But none of these elements seems to work as a port address in Chirp. (I'm sure I'm missing something really obvious...any help is appreciated.)

Edgeport/1:
Product ID: 0x0215
Vendor ID: 0x1608 (Inside Out Networks
Version: 0.01
Speed: Up to 12 Mb/sec
Manufacturer: Inside Out Networks
Location ID: 0xfd136000 / 6
Current Available (mA): 500
Current Required (mA): Unknown (Device has not been configured)