Entity Registration module: Enable registration by default or programmatically
/**
* Implements hook_entity_insert().
*/
function registration_entity_insert($entity, $entity_type) {
  $registration_type = registration_get_entity_registration_type($entity_type, $entity);
  if ($registration_type !== FALSE) {
list($entity_id) = entity_extract_ids($entity_type, $entity);

$settings2 = array(
  'multiple_registrations' =&gt; 0,
  'from address' =&gt; variable_get('site_mail', ini_get('sendmail_from')),
);

$settings = array(
  'status' =&gt; 1,
  'capacity' =&gt; 0,
  'send_reminder' =&gt; 0,
  'settings' =&gt; serialize($settings2),
);

registration_update_entity_settings($entity_type, $entity_id, $settings);
//registration_entity_set_default_settings($entity_type, $entity);
 }
}


References: 

http://drupal.org/node/1430870

http://drupal.org/node/1357280