Make sure you're not running through an external USB hub or anything; connect directly to the computer.
Try disconnecting the cable, and then running:
user@host:~$ dmesg
(a ton of system messages should stream by)
Then reconnect your cable, turn on radio/volume up (shouldn't be necessary for dmesg output, but might as well do it now), and run the same command:
user@host:~$ dmesg
(all of the previous system messages, and hopefully something new indicating a usb->serial device like below)
[430941.789785] usb 2-1.7: new full-speed USB device number 49 using ehci-pci
[430941.883258] usb 2-1.7: New USB device found, idVendor=067b, idProduct=2303
[430941.883267] usb 2-1.7: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[430941.883272] usb 2-1.7: Product: USB-Serial Controller
[430941.883277] usb 2-1.7: Manufacturer: Prolific Technology Inc.
[430941.883801] pl2303 2-1.7:1.0: pl2303 converter detected
[430941.885384] usb 2-1.7: pl2303 converter now attached to ttyUSB0
Assuming your system registers the usb->serial cable, you should see it in dmesg w/the device it was assigned (/dev/ttyUSB0 in my case -- vdir /dev/ttyUSB* is faster if you just need the device ID, but if vdir isn't showing anything, dmesg might give you a hint as to why).
I had to enter /dev/ttyUSB0 manually in the "Port" section of CHIRP -- it didn't show by default unless I restarted CHIRP after the cable was connected.
-Amber