I wanted to get extra storage for Google Apps. So we could sync our ‘my documents’ folder with Google Apps, mainly as a cloud backup service.
But when I wanted to purchase extra storage, I got an error message saying that This service is not enabled.
So I needed to enable this service first: Browse to your Control Panel (https://www.google.com/a/your-domain) > Organization & users> Settings. And enable User Managed Storage.
More info on Google User Managed Storage cloud service.