bbPress.org

That is with bbPress 1.0 right? the object_type is for 1.0

You get nothing using get_results because it returns an array – use get_var instead

<?php

global $bbdb;

$result=$bbdb->get_var("SELECT SUM(meta_value) FROM bb_meta WHERE object_type='bb_topic' AND meta_key='views'");

echo $result;

?>