To discover the MAC address of your Mac's Ethernet card, first open System Preferences:

Mac System Preferences

Click on Network and the following options will appear:


Mac Network Preferences

Making sure that 'Ethernet' is selected, click on Advanced and a new Window will appear:

Mac Ethernet Settings 

 The string of numbers and letters after Ethernet ID: is the Mac address of the machine.

 These instructions have been tested on Mac OS X 10.5.x.