How can I get the MAC address of my Airport Express to add to my router?

I have unfortunately switched to Comcast as my home ISP and there is a stupid security feature on the Motorola Surfboard Arris Routers which will not allow me to connect my 2TB Airport Extreme to my router without first adding the MAC address to my "Authorized Devices" list. However, if I can't get on the network, how can I find the Mac address? It's the perfect Chicken before the Egg argument. I'm trying to extend my current wireless network but cannot get the device on the network no matter what I try. The connection worked flawlessly with my other ISP setup. The MAC address isn't located anywhere on the device unless it's engraved microscopically? I've had the Airport for over 2 years so I have no idea where the original packaging is. And there's no search feature on Apple's website that will allow me to find it by searching the serial number. Any ideas? This is frustrating.