Capacity php script isn't working for me

I am following thread: https://premium.wpmudev.org/forums/topic/how-to-use-capacity-option

I have put the following script in my functions.php:
function modify_capacity( $capacity, $service_id, $worker_id ) {
if ( 7==$service_id && 13==$worker_id )
return 2;
else
return $capacity;
}
add_filter( 'app_get_capacity', 'modify_capacity', 10, 3 );

But nothing happens. Could someone take a look to se what I am doing wrong?