The hook in register method is right. If you want to check before 1.0.9 is out, download /applications/membersshop/hooks/RegisterHook.php. The _createMember method should look like this;
public static function _createMember( $values, $profileFields, $postBeforeRegister = NULL, &$form )
{
$parent = parent::_createMember( $values, $profileFields, $postBeforeRegister, $form );
It requires 4 parameters.