The Media Access Control (MAC) address is a unique 12-character (e.g. 00:00:00:00:00:0X) identifier for a specific piece of hardware, like the network adapter located in WiFi devices.

The instructions differ slightly depending upon which version you are running:

Find the MAC Address on OS X 10.10 (Yosemite), OS X 10.9 (Mavericks), OS X 10.7 (Lion or Mountain Lion):
If you’re connecting via wireless and need to locate the MAC Address of your Mac Book running Yosemite (10.10), Mavericks (10.9), Lion or Mountain Lion (10.7), follow these steps:

1. Open System Preferences (from the icon in the Dock or from the Apple menu).
   
2. Select Network.
   
3. Select Wi-Fi from the left panel and then click the Advanced button on the bottom right-hand side.
4. Select the Hardware tab. The MAC address is listed as WiFi Address.

Find the MAC Address on OS X 10.5 or 10.6 (Leopard or Snow Leopard):

If connecting via wireless and you need to locate the MAC Address of your Mac Book running Leopard (10.5) or Snow Leopard (10.6), follow these steps:

1. Open System Preferences (from the icon in the Dock or from the Apple menu).
   
2. Select Network.
   
3. Select AirPort from the left pane and then click the Advanced button.
   
4. The MAC address is listed as AirPort ID:
   

Find the MAC Address on OS X 10.4 and Earlier

If connecting via wireless and you need to locate the MAC Address of your Mac Book running OS X 10.4 or earlier, follow these steps:

1. Open System Preferences (from the icon in the Dock or from the Apple menu).
   
2. Select Network.
   
3. Select AirPort from the Show drop-down menu. The MAC address is listed as AirPort ID: