How do you enable AJAX in Heartbeat, such that when you click load older messages, it loads the messages? Currently, when I click the link there is no response whatsoever. Do I need to download any file?
No, you don't need to download any file. There was a recent issue with the refresh functionality that might be applicable to what you are experiencing: In heartbeat.entity.inc, at line 242, the "/js" is missing in front of heartbeat.js so the stream are not refreshing.
drupal_get_path('module', 'heartbeat') . '/heartbeat.js', that line should be
drupal_get_path('module', 'heartbeat') . '/js/heartbeat.js'-