CiviEngage, *at the moment*, is a Drupal module that currently ships with the official CiviCRM Drupal release (see https://github.com/civicrm/civicrm-drupal/tree/7.x-master/modules/civicrm_engage, which ends up installed in sites/all/modules/civicrm/drupal/modules/civicrm_engage). That means you won't find it on drupal.org or any other drupal module site and you will only see it in your CiviCRM installation as an available module if you are running CiviCRM on Drupal.

Having said that... plans are underway to first split the module out of CiviCRM and onto drupal.org so it can be downloaded and installed like any other third party drupal module and eventually be converted into an extensions so it can be used by anyone.

More information is available in this issue:

https://issues.civicrm.org/jira/browse/CRM-16671