Thank you for posting.
Your AirPort Extreme will be assigned an external IP address from your ISP. You should be able to view this address using a website such as http://www.whatismyip.com. FileMaker Go would use this address to connect to your network.
When it attempts this connection, your router must decide which computer to forward it too. In short, you must forward port 5003 to the computer that is sharing the database on your network. There are many guides online for setting up port forwarding on specific routers. To configure port forwarding, you only need the port number (5003) and the local IP address of the "server" on your network.