@Maglok
have you tried doing this:
define('FORUM_ROOT_PATH', '/path/to/your/phpbb3/installation/');
$phpbb_root_path = (defined('PHPBB_ROOT_PATH')) ? PHPBB_ROOT_PATH : FORUM_ROOT_PATH;
To clear things up, do you have a successful installation of PHPBB3 running on the server?
If so, the forum path is in the config.php file of the forum installation directory.
bEz
