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Open the file wp-admin-themer.php and modify check following lines :

/* Modify these variables if you have installed wp-admin-themer in another directory */
define('WPAT_PLUGIN_URI', get_option('siteurl').'/wp-content/plugins/wp-admin-themer-extended/');
define('WPAT_PLUGIN_PATH', $_SERVER['DOCUMENT_ROOT']  . '/wp-content/plugins/wp-admin-themer-extended');

Open the plugin configuration page, which is located under Settings -> WP Admin Themer and verify your settings.