BBM isn't available in the PB App World. It's only available while bridging your phone. Are you saying that you don't have the BBM icon under your Blackberry Bridge tab?
When my bb is not bridged with the pb all the bridged folders r shaded and the bbm icon is not there. When I bridge, icons become highlighted and bbm icon still not there
Posted from my CrackBerry at wapforums.crackberry.com
Check your version of BBM? Did you recently update Bridge? With the new version of bridge, I noticed you had to have BBM version 6 to see the icon.
When all else fails try a reboot. Or try to remove the device from the bluetooth options and re-add it to see if BBM shows up again. If that doesnt work, uninstall and then reinstall BMM.
Search the forums. I've seen talk about this exact issue. Might involve a simple hard reset. Try holding all three top buttons down until your PB turns off and reboots it'self.