When you start up postmaster on the server be sure to add the -i option to the command line. The man page reads, "This enables TCP/IP or Internet domain socket communication. Without this option, only local Unix domain socket communication is possible."
Another step is to set up an entry in pg_hba.conf to allow the external host to connect. hba stand for Host Based Authentication.
Lastly, as Vorick mentioned, the user that connects must have access to the database they are connecting to.