Or install the OS on your computer and run the App loader check the check box for BlackBerry Messenger.
You should just double check the Options -> Advanced Options -> Appliations... Not that I don't trust you I just find it hard to beleive BBM is not in their by default.
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8830 -> 8330 -> 9550 -> 9650
Just think about how far BlackBerries have come from then till now... And what else is coming.