If you read the BSPeter's link I posted before (
http://forum.bsplayer.org/viewtopic.php?t=6206 especially items 2 3 and 4 ) you should have read the way to ask for help here..
If you got a BSplayer message "Unknown file format" you may use GSpot to know which codec is needed to play that file and once you know which codec is needed you'll know which codec to download and install ;)
It's not so hard to do, and if you follow these steps you'll never have problems in playing video :)
Once you got the info as described in items 2, 3 and 4 of BSPeter's post we'll be able to help you furter.. I can't tell you which codec to install if you don't tell me which kind of video you are tring to play..