When no patient is selected, you will be prompted to search for a patient before you can create an appointment. You can create an appointment block by clicking and dragging on a Resource column to create the appointment block. The Appointment Detail window will open, if you have "Bring up Appointment Detail window after creating an Appointment" checked in Preferences > Scheduling > Appointments.
The Acct#, Last Name and First Name fields are all in red, indicating they are required. Enter whatever information you have for the patient. For example, if you know their MacPractice account number, you could enter that, or you could search by first and/or last name. After you have entered the search criteria, press Return on your keyboard. A search window will appear with your results. Select the patient's name, then click OK.
After clicking OK, you will return to the Appointment Detail window, and the patient's information will be filled out. Enter appointment notes or make other changes, then click the Save button to close the Appointment Detail window and save the changes.