You add appointments the same way you add a timesheet. You can click & drag on the calendar or click the New Timesheet Entry button. Before you save, however, make sure you flick the switch at the top of the window to Appointment.
If you're entering time for today or previous days, the switch will be set to Timesheet by default. If you enter time for tomorrow or later days, the switch will be set to Appointment by default.
You can change any timesheet entry to an appointment or vice versa. Appointments are indicated by a grey dot, and are not synced to WorkflowMax.