I answered how you install the photopost software with forum integration. You at no time asked about already running gallery. I would suggest you reinstall the software if you want to integrate with the forum otherwise you can try and hand change settings in the database and config files which is not how things are designed to be done and then you have to worry about file ownership and how users in photopost get put into phpbb etc. That is why you integrate when you install a new photopost. Your opening up a can of worms which you would need to write yourself scripts to add users to phpbb and reassociate userid's in photopost etc.
I am moving this post to the install forum for continued discussion and you may follow this thread to verify yourself as an actual customer.