Click on the Internet icon and then click the Internet Connection tab
Check the setting for Connection Sharing
If you want to set up Port Mapping on the AirPort Extreme, the setting for Connection Sharing should be set to "Share a public IP address", not "Off (Bridge Mode)" that I suspect is where it is likely set.
But the fact that your AirPort Extreme "thinks" it should be in Bridge Mode brings up another point.....
You likely have a gateway device (combination modem/router) that is establishing your Internet connection. If that is the case, then that is the device that should be configured to provice the Port Mapping services, not the AirPort Extreme.
The AirPort Extreme should normally be in Bridge Mode if you have it connected to a gateway, which you probably call your "modem".
If you try to change the setting for Connection Sharing to "Share a public IP address" and you see a conflict message or a message about Double NAT, that confirms that you have a gateway and not a simple modem.
If you want to continue to 'break the rules" then the Double NAT is usually not too serious of an error on most home networks, but it may tend to slow communications down a bit.