Appointment Reminder syncs with Google Calendar in near real-time. So if you add an appointment in Google Calendar (and it has a mobile or email somewhere in the appointment) it will show up in Appointment Reminder straight away.
Try refreshing the page to see if the appointment is now showing in the calendar. If it’s still not there then please contact support and will help you figure out why.
If you go to Calendar > Edit Calendars you can see when the calendar was last synced.