I was able to make this work successfully but only after about an hour of experimentation on the settings.

The main problem I had was that my phpbb install was done for me by my hosting provider's Control Panel system, so I didn't know all the database settings. It would be beneficial if the module provided more troubleshooting feedback, e.g. "Unable to locate host" or "Invalid login" etc. Once I got close to the correct settings I finally got an error message (in the module's display window on my homepage) but I'd already tried about every possible setting at that point. I also didn't know which 'admin' login it needed; the documentation should be clearer that it needs the SQL admin login, not the phpbb login.

Aside from my request that there be better feedback/documentation, I withheld giving this module the highest rating because it stores the back-end login in plain-text. My site's now live yet so I'll probably reinstall it so that Joomla and phpbb share the same db, eliminating the need to enter my SQL login info.