One way to get to the "Add New Appointment" screen is to use the "Add Appointment" button found at the top right of the calendar underneath the main "Appointments" menu:
After clicking the green "Add Appointment" button, you'll be directed to the "Add New Appointment" screen where you can set up either a One Time Appointment or a Recurring Appointment. By default, the selected date and time for the appointment will be the current date and time, although you can override this in finding the time for the appointment you're adding. This is different than when adding an appointment by clicking on a blank spot on the calendar (described below) as that will select the date and time that you clicked on in the Add New Appointment screen.
You can also get to the "Add New Appointment" screen by scrolling through your calendar and clicking on a blank spot. To do this navigate to the main "Appointments" menu and make sure you have the "Calendar" tab open:
Find the date and time that you want to add an appointment for and click on it:
You'll see that when you enter into the Add New Appointment screen that the date and time are pre-filled with your selection from clicking on the calendar:
After you've entered into the Add New Appointment screen, you are all set to start adding One Time or Recurring appointments.