LW's media player itself is only an audio player. So if you launch or preview a video file it means your default video player has been opened to play the file. For xtra codecs see either: http://www.land.ee/d_nimo_codec.html OR http://lists.gpick.com/pages/CoDecs.htm OR http://www.free-codecs.com/
OR I suggest you get a backup player like VLC which has in-built codecs & plays almost everything: http://www.videolan.org/vlc/ (choose nearest mirror site)

If you decided to make VLC your default player then see: Setting Default video app