There is a link to the ACE website under Academic Services. The direct link can be found here:
Once you're on the ACE website, simply click the blue box labeled "Book an Appointment" located in the middle of the page.
From there, a separate screen will pop up that let's you select the type of appointment you need (i.e. writing tutoring, math tutoring, academic advising, etc.)
The names and available times for the appropriate tutor will be highlighted in yellow on the calendar. By clicking a time slot, you can then fill out your name and contact information. You will receive an email confirmation that you have booked the appointment.