The problem is that nothing in WP is loaded if you are trying to load the content of a specific PHP file using AJAX.
In order to have access to all WP and plugin function you must make all request through /wp-admin/admin-ajax.php, setting the required “action” and creating the needed WP filters rather than requesting a specific file. This page explains AJAX in WP, it says AJAX in Plugins, but the information is relevant to all AJAX requests in WP.
