Here is how to install or reinstall the blackberry messenger.
1-Plug your Blackberry into your pc and start up the blackberry desktop software:
2-Make sure you have the latest desktop software for it to work at its best
3-Start the desktop manager and go to application loader, start the application loader, and you should see a list of applications.
4-Scroll trough the list until you find Blackberry Messenger. Make sure its checked, and hit next, and follow the directions there.