OWL CM160 Energy Monitor and RFXTRX

I’ve had an OWL CM160 Energy Monitor for a while but until today I didn’t realise that it had a USB interface. I installed RFXtrx and specified /dev/ttyUSB0 for the device, but although the RFXtrx integration starts without error I don’t see any new entities and the integration card is blank. Am I missing something really simple?