When I look at the update page for the contents I can see that the iRite editor include code looks like this:
<?php require_once("../WA_iRite/WARichEditorPHP.php"); ?>
The error message you are getting indicates that there is a problem with the include path you have on this page. Please adjust this path accordingly so that the correct include file is referenced when the page loads. This should resolve the error.
The js file referenced is contained inside of the iRite folder in your site. You may also want to upload the iRite folder for the site again if it is not found on the server.