Just an update on this error. This mod seems to have a typo in the classes file it will not add new products to your feed that you add after you install this mod.
The error can be found in the file: includes/classes/rss_feed.php
find around line 383 :
PHP Code:
          if ($time == false || (time() - filemtime(DIR_FS_RSSFEED_CACHE . '/' . $zv_file)) < $time) { 
Replace with:
PHP Code:
          if ($time == false || (time() - filemtime(DIR_FS_RSSFEED_CACHE . '/' . $zv_file)) > $time) {