You can’t. You have to write your own query like the one below.
$the_query = new WP_Query( array( 'meta_key' => 'ratings_average', 'orderby' => 'meta_value_num', 'order' => 'DESC', 'category__not_in' => array( 3 ) ) );
// The Loop
if ( $the_query->have_posts() ) {
echo '<ul>';
while ( $the_query->have_posts() ) {
$the_query->the_post();
echo '<li>' . get_the_title() . '</li>';
}
echo '</ul>';
} else {
// no posts found
}
/* Restore original Post Data */
wp_reset_postdata();
See http://codex.wordpress.org/Class_Reference/WP_Query