- From the Apple in the upper left hand corner, choose System Preferences System Preferences
- Click the Network Network icon
- Select Ethernet/Thunderbolt Ethernet for your Ethernet MAC or WiFi/Airport for your Wireless MAC
- Click on the Advanced button
- Click on the Hardware or Ethernet tab
- Your MAC address will be at the top of this window as your Ethernet ID.
See this wiki for a more detailed procedure with screenshots.