To reach the AirPort port mapping section on the router, you must use 'Airport Utility' software on either the MAC computer or the Windows Computer. You MUST also confirm that the AirPort router is on the "Share a public IP address" under the "INTERNET TAB."

If you are on any other settings such as "Distribute a range of IP addresses" or "Bridge" mode you will not be able to setup port forwarding within the router.  The image below show the correct setting to use on the airport utility router.