In your DokuWiki installation, create a folder lib/plugins/authfluxbb/ and create the two files auth.php and plugin.info.txt in it.
In your wiki's conf/local.protected.php (create this file if it does not exist), add the following lines:
$conf['authtype'] = 'authfluxbb';
$conf['superuser'] = '@Administrators,@Moderators';
// Make sure stripping slashes doesn't clash with the forums.