Can I see a copy of the page that isn't working? Maybe get FTP access?
It looks like the WA_CMS.php page isn't included on the top of the page like it needs to be, or the addtosite.php file that will automatically include the WA_CMS.php page.
How is the cms file referenced on the site that it isn't working on? It may just be missing that include file on the top of the page.