I had the same problem. You need to go to the cable manufacturers website and get the drivers for the cable.
If you are using mapsource you need to have upload to device/ save to device set to serial port and not usb. Mine ended up using serial port 4 with the usb adapter.
Another thing I noticed is it only works on 1 USB port. It will not find it if I have it plugged into the one of the other 3.