What OS are you running?
Try upgrading your OS (4.5 is the top for that phone IIRC) and that will add BBM automatically on install, should work just fine, if not, go to the official blackberry website, under softwares, search for the BBM and download it again (replacing your "current" version).
Serge - follow me on Twitter @SergioSeminario
HP Envy 15
BB Curve 8520
iPod Touch 4G