When you open the Server Console and click "databases" do you see your databases listed?
Are they shown as "open"?
From the console select help and find the list of ports that should be open. Do you have a firewall up that blocks those ports?
That'll at least get you started.
Also, Under Sharing | Filemaker Network...
Make sure that "All users" is selected.
Also make sure that Bonjour is installed properly. Have you tried specifying the IP address explicitly?