Go to the apple icon in the top left corner of any program. Right click, and go to “About This Mac”.
In the “About This Mac” Window that pops up, click on the “More Info” button.
In the “System Profiler” window that opens, choose “Network” from the list of options in the left vertical panel. By default, the airport should be the selected device, so you'll need to select “Ethernet” from the “Active Services” window. Once you have “Ethernet” selected, you need to scroll down to the bottom of the bottom right panel of the window. You are looking for “Ethernet – MAC address”.