In the web admin, go to the CM11 device, under "Device data" I have 2 choices there: "/dev/ttyS0" and "pci0000:00/0000:00:13.1+2", I'm using the latter which is the USB port I plugged it in (I checked with "lspci" to see what it actually was). And that's all I had to do for that. the USB cable (with convertor box in the middle) that I have doesn't seem to require any additional settings or emulate a serial port (I doubt it's the ttyS0 I'm seeing).
I have the serial cable as well but my mobo only has an internal connector for rs232 but no cable+connector so I had little choice. I'm thinking of hooking it up to a gc100 (if I can ever get the distributor here to actually sell me one
) eventually. Anyway, good luck!