Launch the AirPort Utility from your Applications:Utilities folder.
Select your Base Station from the list on the left and then click the Manual Setup button.
Click to select the Advanced tab from the top of the Manual Setup window.
Click to select Port Mapping from the top of the Advanced Settings.
Click the + button to add a new port forward. The plus button can be found under the list of existing port forwards.
Enter the settings for the port forward rule. The public ports are the ports that external computers will use to connect. The Private IP Address is the IP Address of the computer in your network that you would like the public ports to forward to. The private ports are the ports on your local computer that you would like the public ports to point to. ie so an external computer could think its connecting to port 80 on your computer but the router would resolve it to a different port of your choice.
Once you have inputed the correct settings for your port forward rule, click the Continue button.
Enter a description for your port forward rule.
If you like you could enable Bonjour to broadcast the rule globally but we will leave this alone for now.
Click the Done button to save your new rule.
Now press the Update button at the bottom right hand corner to update your router with the new port forwarding rule. Please note this will reboot your AirPort Extreme Base Station!