We need some more information on your network. Where is the file that you are trying to point FM Go to? Is it on the same local area network? If so, you can use bonjour to find it. Or just the local IP. Is the Airport Extreme acting as the DHCP server? If not, is the ISP the DHCP server? If your server is inside the local area network, then you need to have the router forward the ports or do network address translation to the server for whatever the public IP is of the router. Do you have a public IP? If not, are you using dynamic DNS routing like noip.org? We just need to know a lot more about your network to answer your question.