OSX (Apple) - from the Apple menu, select System Preferences... . From the View menu in System Preferences, select Network.
In the left column of the Network preference window that opens, click the name of your connection (e.g., Wi-Fi, AirPort, Ethernet, Built-in Ethernet). Click the Advanced... button, and in the sheet that drops down, click the Hardware, Ethernet, or AirPort tab. The address is the string of letters and numbers next to "MAC Address:", "Ethernet ID:", or "AirPort ID:".