Before going into media player to activate the sharing go onto your PS3 and turn on the Media Server Connection- you'll find this in Network Settings.
Once that is done you can turn on the media sharing in media player. Below this you should now see an icon for the PS3 in the device window. Click the PS3 icon once (may not say PS3 but it should be the only other device there) and hit the Allow button. Hit OK and that should do it- when you try a media search on the PS3 it should see the folders you have your MP3s, videos, etc. on.