I believe you are looking for the config.php file . you have to access it directly on the server using ftp. download the file, copy it. then edit it and upload it back to the server.
purge the cache and it should then show you whatever database you put in the file.
It seems that phpbb didn't have permissions to write to the directory. This explains why I couldn't find the config.php file. Reinstalling with the correct file permissions fixed the problem.