Firstly, assign a manual address to the IP camera. For example, my router is 10.0.0.1 and my IP camera address is 10.0.0.100, Gateway is 10.0.0.1, subnet is 255.255.255.0 and DNS will be 10.0.0.1.
Set the camera port to 8080 - Save configuration and restart.
Once rebooted, open up your cameras web page - in this case it will be http://10.0.0.100:8080 - confirm you can access the page.
Next go to the airport extreme firewall page and enter an IPV4 port mapping.
Public TCP port will be 8080.
Private IP address will be in this case 10.0.0.100
Private TCP ports will be 8080
Save, and wait for the airport extreme to restart.
Now, assuming you know your external IP address, you will need to check from outside your network - a 3G connection is useful - Go to my.ext.ip.addr:8080 and you should get the camera login page.
Why do you need to use another connection? I don't know if the airport extreme has the ability to recognise your external internet address as an internal connection - you can try without but it may not work.
Once you get the page - You have got through to the camera, but the camera will be totally unsecure. SSL is highly recommended, if not paramount - but that's another question.
In some cases I have experienced bridged modems still blocking ports, and some internet providers have built in firewalls which need reconfiguring or switching off.
With standard home equipment and home internet connections, it's always a little troublesome and rarely works first time.
I also find it far easier and more secure to use a service like http://www.ispyconnect.com/ which runs a program on your computer to do all the security and part of the configuration for you.
Good luck!
