the loopback address works for the host because it's a fixed address meaning "this PC" (so whichever PC you use, it will connect to itself) to connect to a different one on the LAN you have to use its LAN IP Address, you can either calculate the correct hash for the room based on the LAN IP of the host, or you can cheat and in winmx type RoomName_IP:Port (for example Some Room_192.168.0.2:3 if the hosting PC was on 192.168.0.2 and using port 3) - and winmx will then join the room (plus give you the correct hash to use for any other programs you want to join with that don't support IP:Port)