How to allow port 80? (NAT)
the article currently titled "AirPort Utility 6.x: Set NAT options for your base station or AirPort Time Capsule" and currently available at http://support.apple.com/kb/PH5103 has:



To set NAT options, your base station or AirPort Time Capsule must be set up to share its Internet connection using DHCP and NAT.


Open AirPort Utility, located in the Utilities folder in the Applications folder. Select the base station you want to set up, then click Edit. Enter the password if necessary.
Click Network and then choose DHCP and NAT from the Router Mode pop-up menu.
Select Enable NAT Port Mapping Protocol.

NAT Port Mapping Protocol (NAT-PMP) is an Internet Engineering Task Force Internet Draft, an alternative to the more common

Universal Plug and Play (UPnP) protocol implemented in many NAT routers. NAT-PMP allows a computer in a private network (behind a NAT router) to automatically configure the router to allow clients outside the private network to contact this computer.

Included in the protocol is a method for retrieving the public IP address of a NAT gateway, which allows a client to make this public IP address and port number known to peers that may wish to communicate with it. This protocol is implemented in current Apple products, including Mac OS X 10.4 or later, AirPort Extreme and AirPort Express networking products, AirPort Time Capsule, and Bonjour for Windows.
Select “Enable default host,” and enter the IP address of the host.

A default host is a computer on your network that is exposed to the Internet and receives all inbound traffic. A default host may be useful if you use a computer on your AirPort network to play network games, or want to route all Internet traffic through a single computer.


You can also set up port mapping to direct network traffic to a specific computer on your network.



the article currently titled "AirPort Utility software and firmware downloads" and currently available at http://support.apple.com/kb/ht1998 tells you where to get Airport Utility.