I have no idea about this mod here, but the error above is due to the define DIR_FS_DOCUMENT_ROOT not being set correctly. This ought to be set to your store's root directory usually, like this:
define('DIR_FS_DOCUMENT_ROOT', '/home/username/public_html/');
OR:
define('DIR_FS_DOCUMENT_ROOT', '/home/username/public_html/catalog/');
etc..
alternatively, try:
define('DIR_FS_DOCUMENT_ROOT', $_SERVER['DOCUMENT_ROOT']);
as the original docs are probably out of date