Just working from memory here but you need to have compatible (with home sharing) versions of iTunes on both computers (each computer that the other device syncs with). I believe that Homesharing came out with version 9.0 of iTunes. You will need to turn on sharing in preferences and select what you would like to share.
Once that is complete, click on the HomeSharing menu item which should appear just below the Store menu group. You will need to enable home sharing on each iTunes and then you will need to sync the two shared libraries via your network. music, movies, TV shows, Podcasts, Applications, Ringtones and Playlists can all be sync'd individually between the two computers. Highlight what you want from the shared library and select Import. You can choose to display all items or only items that are not currently in your library.
More from Apple here:
http://www.apple.com/support/ilife/t...nes/it6-1.html