All BlackBerry devices support the XVID/MPEG-4 (.avi; 400Kbps) file format; which means that any file in that format exported to your device's external memory card will work under the stock media player without any additional fine tuning. Most videos these days, however, can be found in a wide variety of formats; which simply means that they will need to be transcoded, or converted, to the supported format.
There's a wide selection of programs available that will convert video files stored on your computer to the proper format. If you would like to find a program on your own, again just do a quick search in a search engine and browse through the results. Otherwise, here are some known programs that work well with most BlackBerry devices and will allow you to convert movies for BlackBerry phones.
The official BlackBerry site lists the Roxio Media Manager Software as its definite conversion tool. however, third party applications are also supported for formatting files.
Some of those third party programs include:
For users interested in a non-pay converter that works well I would recommend the Any Video Converter which supports a wide selection of formats and better yet; it's free! You can find the official Any Video Converter website here.
You will have to refer to the individual program documentation on converting the actual files; but the process is not difficult at all.
Users can also utilize the paid application BlackBerry Media Studio which essentially converts DVD movies into a compatible BlackBerry video format.
With these conversion packages you'll be watching movie on your BlackBerry in no time.