How to make your actions known to Rules

/**
 * Implements hook_rules_action_info(). See http://drupal.org/node/878928 for more info.
 */
function my_rules_action_info() {
  return array(
    'my_callback' => array(
      'label' => t('My custom function'),
      'group' => t('My action collection'),

      'parameter' => array(
        'account' => array(
          'type' => 'user',
          'label' => t('Recipient'),
        ),
        'message' => array(
          'type' => 'text',
          'label' => t('Message'),
        ),
      ),

    ),
  );
}

/**
 * My action function
 */
function my_callback($account, $message) {
  // Stuff goes here
}