There are two ways to adjust video properties.
1. Open a movie, go to options, filters and select the video decoder it's used. If is DivX and you don't have the sliders shown, then uncheck the YUV extended mode and restart the player. You must have now the sliders. If is xvid and you don't have the slider for brightness then install 0,91 or 0.92 version.
http://forum.bsplayer.org/viewtopic.php?t=3042
2. Open the movie, go to video, color controls and adjust them.