mobile media player does not work !!!help

my nokia mobile media player not working or ive downloaded wrong formats !!! im using smartmovie player and converter etc and when tring to use mobile nokia player it does not play the converted files, do you no why, and it shows the files within the player its self but when i click on play i just get a blank grey scrreen and no audio, any suggestions would be good, ie is smartmovie player better than mobile player and whats formats does mobile player need and what converters to use....thannkyou