In order to determine the MAC address of an Instant Replay 2 or DigiCart E device, you need to utilize a network scanning tool such as Advanced IP Scanner https://www.advanced-ip-scanner.com/
To determine the MAC address, attach the audio player to a network and either using DHCP or assigning an IP address manually.
With a PC on the same network as the audio device, install and run Advanced IP Scanner or another network scanning tool. Enter the IP address of the audio device into the IP scanning program on the PC an start a scan. The tool should report back the IP address of the equipment.