The css error is on the following line in embed-rss/cets_EmbedRSS.php:
wp_enqueue_style( ‘cets-jquery-ui’, plugins_url(‘/embed_rss/lib/jquery-ui/cets-jquery-ui.css’), array(), $this->version, ‘screen’ );
There is a type-o. The directory ’embed_rss’ is supposed to reference ’embed-rss’
Just change that and the editor window will display correctly.
-D