This is from the ticketbus site:
ADO express bus departures from Playa at 09:20, 10:50, 11:50, 13:20, 14:35, and there are more before & after these times.
http://www.ticketbus.com.mx/There is an English version of the site -- look at the top right side of the page.
On the left side, you have to select the bus terminals. For the "From" terminal, don't select "Playa del Carmen Alterna" even though that's the terminal from which the buses depart. Select "Playa del Carmen, Qroo".
When you get the page of schedules, notice that the schedules are arranged under separate tabs for Dawn, Morning, Afternoon and Night. You have to click on all the tabs one by one to see all the departure times for the whole day.