If you have booked an appointment with a club that supports mobile access for appointment bookings you will see the access option when tapping on your booking from the app home screen.

You will only see access available from 15 mins before the appointment starts, you will not be able to access prior. If you have booked an appointment with less than 15 mins until the start time access will be granted within 5 mins of making the booking.

If you have never accessed the club with the members app before, you will need to grant some permissions to access the club. You can read how to setup access here