Google Apps supports the concept of multiple calendars associated with your Google account. Every account starts with a single calendar - this is your primary calendar. Within Google you can also create additional calendars.

Appointment.one currently supports only the primary calendar within your Google Apps account, and does support these additional calendars.

You can still make use of multiple calendars with Appointment.one by making sure that each calendar is associated with a different Google Apps (or other provider) account. This is most commonly done with provide a more accurate view into free/busy time.