try this, not guaranteeing it will work, but im trying to help a friend with the same problem
1. completely delete the BBM 5.0 version from your phone
2. after deletion, let your berry reboot (takes a few min)
3. now open the browser on your phone, go to www.blackberry.com/send/messenger
4. on that page, click DOWNLOAD
5. Before you download and install, verify that the version is lower than 5.0
Should work. Best of luck! I will report back findings after my friend has tried this out.