I have more than one.. wireless and a nic.. but if you don't have anything plugged into nic and wireless disabled.. I get that error.. it seems that you have to have a valid IP before you can get the mac..
This library binds to the IP list in order to return the MAC, that is why it won't return the MAC of a disconnected card. I will investigate the mac via interface class in java and add that one to this library soon as I can.
Thanks.