Looks like you will have to use
$meta = get_post_meta($post->ID, '_selected_post', true);
echo $meta;
The registered post type is hidden using the _ and it’s name is defined at the line :
$this->meta_key = "_selected_{$this->SELECT_POST_TYPE}";
Where SELECT_POST_TYPE is defined as ‘post’ at the line :
var $SELECT_POST_TYPE = 'post';
Hope it helps.