Place the function call wherever you want the output to appear.
<?php mdv_comment_count(); ?>

The function accepts one optional parameter, which allows you to find the comment count for a particular post when it is used inside the "the loop". If you do not pass a parameter to the function, it defaults to true, counting ALL comments for a WP install. If you call the function like <?php mdv_comment_count(false); ?> it will count the comments for the post that "the loop" is working on.