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Take whatever video file you want to load into your BB, and copy it into the mplayer folder.
Make a mental note of the exact name of the file along with its extension. For example, if my Quicktime video was called "phone", the full name would be "phone.mov". Windows media player files are .wmv Others are like .mpeg etc.
Now take a look at this stuff:
mencoder -vf scale=240:-2 example.mov -o temp.avi -of avi -ovc lavc -oac lavc -lavcopts vcodec=mpeg4:vbitrate=320:acodec=mp3:abitrate=64
See example.mov? Replace that with the full name of your file. After that, highlight and copy everything you see there, from mencoder to 64. Open up the black mplayer command box, and right click>paste. Hit enter and it should begin converting. The converted file will be in the same folder, except in .avi format. You may now load that into your BB and it should work like a charm.
I just did this today. The quality is amazing.