From desktop, click on the apple icon in the top-left corner, and select About this Mac.
A small window will appear, click on More Info...
A slightly larger window will appear, with a picture of your Mac. Click on the System Reportbutton.
About half-way down the list on the left, click on Network.
From the blue and white striped pane at the top, click on the method of connection for which you would like the MAC address. This will either be Ethernet or Wi-Fi.
Many details will appear in the bottom pane. Look for the heading labeled Ethernet (it's the same heading name even if you selected "Wi-Fi") near the bottom. The MAC Address is listed underneath that heading. In this example, the MAC address is 00:22:41:24:3b:41.