The listener operates as a TCP/IP server which is listening on a port number. The application that you wish to receive messages from must be configured to connect via TCP/IP as a client to this port on the host that the listener is running on.

You will need to ensure that the port you are trying to listen on is not being used by any other applications running on your machine. It's not possible for other applications to listen at the same time to the stream of HL7 messages that the listener is receiving.