You have to have the same version of MJ on both PC's. You start Media Server on one (the server) and you listen to music on another PC (the client) by connecting to the server from MJ.
More from MJ Help:
Using Media Server
What is Media Server?
Media Server provides on-demand streaming of your music. Rip once, play anywhere. Listen to the music on your home computer from work, from a friend's house, or on your laptop from anywhere else in the house.
Start Media Server
From the Windows Start menu, go to Programs > Media Juekebox > Media Server; or
From within Media Jukebox, go to Tools > Media Server.
When Media Server opens, go to it's File Menu and select "Share".
Stop Media Server
To stop Media Server go to File > Stop Sharing.
Connect to a Server
Start Media Jukebox and go to Tools > Media Server > Connect or Connect To Last if you want to reconnect to the last host you used.
Type in the host name or an IP address followed by a colon and the port number if you wish to specify a port (the default is 80) You can also select the arrow on the right for a drop-down list of the most recent hosts to which you connected. Here are some examples of what valid host names might look like:
music.jriver.com
music.jriver.com:8931
198.121.98.76
198.121.98.76:8931
MyComputerName
Disconnect from a Server
In Media Jukebox go to Tools > Media Server > Disconnect.
View Statistics
In Media Server go to File > Statistics.
This will display a list of files that have been shared. You can reset it by selecting File > Statistics > Reset.
Enable Authentication
You can set a username and password here, so that when there is a connection to your server, the user will have to type in the username and password before accessing your library.
In Media Server go to Settings > Options and click on "enable authentication". You will have to provide a username and password.
Settings
In Media Server go to "Settings" on the Menu bar. From here you can display or hide the Status window, or select "Options" to open the following menu items:
General. From this menu you can enable authentication, set Media Server to automatically start on Window's startup, and set it to automatically start sharing when the program starts.
Network. Select automatic port selection (the default is 80), or manually input the port. You can also reset "current maximum bandwidth".
Library. Set the library you would like to share.