Google apps Calendar Settings by default does not allow users to share their calendar externally. Here are the steps for how to change the default settings of Google apps Calender and enable external sharing including iCal private address.
- Login to Google Apps Admin Console.
- Click Apps > Google Apps > Calendar.
- Under Share settings, change the default to one of the following options:
Share all information, and outsiders can change calendars or
Share all information, and allow managing of calendars
- Click Save Changes.
- Once the Google Apps Calendar sharing settings have been completed, the Private Address links for ICAL and XML should appear.