We recently upgraded to 4.6.3, and the page message templates live at seems to have changed: the link in our menu bar (https://secularstudents.org/civicrm/admin/messageTemplates?reset=1) is returning a page not found error. All of our message templates still exist. Any suggestions on where to find them so I can change the navigation link to work properly?
CiviCRM 4.6 has a new permission to edit templates separate from "Administer CiviCRM", and I'm almost certain the issue you're having is that you need to add permissions post-upgrade.
From your URL, I'm pretty sure you're running Drupal (and I think I knew that anyway!) so go to http://secularstudents.org/admin/people, select the "Permissions" tab, and locate the permission called "Edit CiviCRM message templates". Check that off for the appropriate roles and press "Submit" at the bottom of the page, the problem should then be solved.