Vapilot is correct.
In System Profiler, you have to make sure that the "Airport Card" line is highlighted. Scroll down to the bottom in the lower field where the information is contained. Confusingly, the MAC number comes under a sub-section called "Ethernet". This is the Airport MAC number. If you clicked on "Built-in Ethernet" at the top, the MAC number would be different. Just make sure you're clicking the right network connection in the top field on System Profiler>Network.
Applemacster