Are both computers on the same local network - no router between them? If not, then there's no way to query the MAC address as far as I know; the client would have to send that identification information. However, that might not be a bad approach - is there any reason you can't have the client send a unique identifier, such as MAC address, when it first connects to the server?
If the computers are on the same local network, then you can use System Exec to run the arp command. On Windows, the command line would be "arp -a [ip address of remote system]". You'll then need to parse the returned text to get the MAC address. Try it out at a Windows command prompt to see the format of the response. This only works if the computers have already established a connection; if not, then the MAC address isn't cached in the ARP table and you'll have to ping the IP address first.
