This error usually indicates TCP/IP communication problem with the specified host and/or port. You need to find out the following:
1) What is the machine name (host) that FileMaker is running on? If it is localhost then this is unlikely to be the problem and most probably the problem is with the port (see below). If it is not localhost then check that you specified the machine name correctly in the URL
2) Connection to the specified port is refused. Did you specify any port in the URL string? If you did, check that FileMaker is indeed listening on this port. If you didn't specify the port this usually means that the default port is used. Check the documentation to find out the default FileMaker port. Then make sure that this port is not blocked by firewalls and that FileMaker is indeed listening on this port (maybe it needs to be configured to listen on this port first).
You can easily find out which applications is listening on which ports by doing a port scan or by running a good program called ActivePorts that you can download from the Internet for free.
Just read FM documentation and it looks like FileMaker should be listening on port 2399. Please check if this is the case by running port scan or Active Ports. Please also check that this port is not blocked by firewalls.