There are two ways to determine this. The direct method and the indirect method.
The direct method requires that you know the IP address of the unit and have a physical Ethernet connection to the unit. From a PC (Windows) open a command dialog (cmd entered in the Start button 'Search program and files' box). From the command dialog you may be familiar with the "ping" function. Another useful utility is the "arp" function. In this case you can use it to get the MAC address back from the unit.
In the command dialog enter the following: arp -a 192.168.10.43
(192.168.10.43 indicates the IP address of the unit so replace with the IP address of your unit).
The response will be in hexadecimal and listed as Physical Address. See screen shot for ping and arp examples.
The indirect method is to contact us at firstname.lastname@example.org (or call 800-358-9068) with the serial number from the unit. Our Applications Engineering (technical support) group can look up that serial number to see what MAC address was assigned.