BuddyPress.org

Just Place this on your template files or header file or where you want to display it

<?php if ( is_user_logged_in() ) : ?>

 <a href="<?php echo bp_core_get_user_domain(bp_loggedin_user_id()) ?>messages/" title="Messages" data-toggle="tooltip" >

<i class="fa fa-envelope-open-o" aria-hidden="true"></i> //Font Awesome if you want the envelope icon
<?php echo bp_get_total_unread_messages_count( bp_loggedin_user_id() ) ?>

 </a>

<?php endif ; ?>