When you load an appointment into Appointment Reminder you need set the time before an appointment when the reminder will be sent. (most people use 24 hours)

You'll want to set the default "when to send" time to the most common time you use. This will save you having to edit this every time you create a new reminder, also reduces errors.

When you're next in an Appointment Reminder click the settings button.
 
Inline image 1


Then check credentials.
 
Inline image 2


Then change when to send to the desired time.


Inline image 3
 
Next time you load a Appointment Reminder the default value will be selected.