Click the top menu Apple icon.
Click ‘About This Mac’
Click ‘More Info…’
The next click depend OS with OSX you are using-
In Mountain Lion, Click ‘System Report’ then click ‘Network’ and ‘Ethernet’
The MAC address will be next to Ethernet in the lower right pane.
In other OSX versions, Click ‘Network’ and the MAC address will be listed in the right pane.