BuddyPress.org

— if you want just a nav item for <i>inbox messages</i>, than a simple slug would be easy to add.

If you want the entire notification tab there… same type of thing as before,

function bp_adminbar_notifications_menu() {
global $bp;

if ( !is_user_logged_in() )
return false;

echo '<li id="bp-adminbar-notifications-menu"><a>loggedin_user->domain . '">';
_e( 'Notifications', 'buddypress' );

if ( $notifications = bp_core_get_notifications_for_user( $bp->loggedin_user->id ) ) { ?>
<span></span>
<?php