What is your BlackBerry OS version?
Kindly be informed that the video playback is supported by third party applications like Dollars 5 medialayer only from v4.2.1 - it is easy and good to upgrade your OS if you had not done it before - read this thread for more details -
http://www.blackberryforums.com/gene...y-release.html
If you already have 4.2.1 and still face problems - it would be kind of you to detail on the problem - like what error do you face? video encoding type? which tool you used for creating the video? this information should aid us help you fast.
You can also try some of our demo videos from
http://dollars5.com/QA/roadkill.avi and
http://dollars5.com/QA/sweety.avi to compare and contrast Blackberry bundled media player and Dollars 5 mediaPlayer.