So, the bbPress bulletin board plug in currently available for WordPress does not have its own PM unit. The suggestion by the bbPress developers is to use BuddyPress, the social networking plug in, but that is like killing a fly with a sledgehammer. Buddypress is just as resource intensive as bbPress itself, not to mention awkward, poorly designed and full of conflicts when it comes to theme integration.
I've gotten around that by implementing your excellent plug in and partly integrating it with bbPress.
That's the page where I did a little minor integration with bbPress to put a link over to your PM plug in.
It would be AWESOME if you could tweak this to run within the bbPress framework. I don't think it would be all that hard to get the PM box to display within the profile screen like the other bbPress profile mechanics. If I was a little stronger with php I would try to do it myself.
But if you could integrate your PM system onto that bbPress profile page, AND implement a pop-up notice message and maybe add some code for users to see an inbox counter on their main browsing page, I bet your downloads would skyrocket. And we would be eternally grateful. I know a lot of people who have complained in the bbPress forums about the lack of a PM system but like I said, those guys love BuddyPress, which is a bad solution.