Just make sure bbm is deleted off your bb and plug it into your pc. Then run app loader and you should see bbm on your list of apps. Check mark it and it will reinstall it. This is assuming you have the current os that is on your phone installed on your pc is as well?
ok delete the one from your phones application list... delete any BBM's installed on the pc, Open desktopmanager>applicationloader>updatesoftware>Op tions>it should auto select anything on the phone already, and you will have to check BBM, if you have not I would back up your phone first. To do that open Desktop manager>backup/restore>backup>voila