Some Google Apps accounts do not automatically have the setting for private calendar sharing, you will need to enable this to access a private ICS feed for your Calendar. For the protection of your privacy and the privacy of your clients, we highly recommend keeping your Google Calendar private.
To change this setting, follow the steps below:
1. Login to your Google Apps Admin console and go to the Settings of your account.
2. Select Manage this domain.
3. Go to Apps, Google Apps and then Calendar.
4. Under Sharing settings, select either "Share all information, and outsiders can change calendars" or "Share all information, and allow managing of calendars".
5. Save the changes.
This will enable your Private iCal Address to show in your settings.