I found this blog very helpful today, when looking for a way to find XBox's MAC address. When XBox is turned on, it kills my home network. So, I put it in DMZ. Now the XBox stopped working, and when it came back from Microsoft after repair, it had a different network card. so, I had to find its new MAc address, and here how it works:
Ping a remote device that you want to find the MAC address (for example: PING 192.168.0.1).
Type "ARP -A", and press Enter.
Interestingly, ping on XBox didn't work, the host didn't reply. However, ARP -A command returned the result correctly. Nice.