I'm adding mp4 video file via ovi suite but after disconnecting it i can't see or play it at media player on my n900.
Waiting for your help

uhhh, what does that mean - it has converted it, or just copied it to the device?
try installing "extra-decoders" and then go read this:
http://code.google.com/p/extra-decod...roubleshooting
try typing your next question right into the search thing at the top right...