So are you trying to access the external url from inside your network? The fact you are prompted for credentials indicates that forms-based authentication is not enabled. Make sure you save your changes using the IFD tool and then perform an iisreset.
The second error, that says the page cannot be displayed when attempting to access the external url, could be many things and the HTTP status code should be at the bottom of the page. From the external system, perform a ping to the IFD url and make sure the DNS is set up correctly on the outside.