You can't
. LW's media player doesn't support video. It'll only play certain types of audio files(mp3's). The player itself isn't very reliable, it tends to skip alot and is really only good for previewing files.
When you try to play video or other files LW player won't play, whatever the default player you have set for that file type is used. You can change the default player from WMP to something else.
Open the folder with your files in it(in windows, not LW). Right click a file and click on Properties > then click on "Change" > choose a different player from the list > click OK > Apply > OK. Do that for each file type you want to change.
How to set the Default applications
CODECs list for Movies/Alternate players / Video help forums/ video-mp3 fixers