Is the AirPort Extreme set up and operating at the present time......even if it might not have an Internet connection?
If yes......open AirPort Utility on your Mac.......(Finder > Applications > Utilities > AirPort Utility)
Hold down the option key on your Mac while you double-click on the picture of the AirPort Extreme
The next page will display a summary of the settings
The AirPort Extreme actually has 3 different MAC Addresses.......Ethernet , WiFi 2.4 GHz and WiFi 5 GHz. if your AirPort Extreme is connected to a modem using an Ethernet cable, then the Ethernet ID is the MAC Address.
If the AirPort Extreme has not been set up yet at all......
Open AirPort Utility as above, then click the File menu in the upper left corner of the screen near the Apple icon
Click Configure Other
For Address, enter 10.0.1.1
A password is normally not needed, but if you are asked for a Password, use the word
public
Click OK, and the summary page of settings will appear. In most cases, since the AirPort Extreme connects to a modem using an Ethernet connection, you would want to use the Ethernet ID as the MAC Address.
If you are still having difficulty, you have 90 days of free phone support from Apple. Give them a call, and maybe they can explain why what should be a very simple task is so difficult.
Contact - Official Apple Support