External class (migration class) not found
The class name in your include file is MhtAuthorDestinationMigration, but you list its class name in hook_migrate_api as MhtAuthorDestination. Uncomment the line and try:

$api = array(
  'api' =&gt; 2,
  'migrations' =&gt; array(
    'MhtAuthorDestination' =&gt; array('class_name' =&gt; 'MhtAuthorDestinationMigration'),
  ),
);

return $api;