I know what you mean, BSprout. That site usually works well, but I was using it yesterday, and it wasn't working at all on Firefox or IE!
In addition to what Lan said about the tabs, it doesn’t seem to work so well when you look up times more than a couple of weeks further on from today’s date. But the schedules don’t vary much, and the schedules for one day will probably be the same as every other day of the week.
Here's the link to the ADO site:http://www.adogl.com.mx
And here is some information from my website:
At the Chetumal ADO terminal, get an ADO Express bus to Playa del Carmen; departure times are 06:30, 08:30, 10:30, 12:30, 13:30, 14:30, 15:30, 16:30, 18:30, 20:00 & 00:45. The trip takes about 4.5 hours and costs MX$204 for Primera Class (as at June 2009).
The bus arrives at the bus terminal at Avenida 20 and Calle 12 (Terminal Alterna). If you want to continue to Cancun airport immediately, get a taxi (MX$20 or 30) or walk to the other bus terminal at Quinta Avenida and Avenida Juarez where you get the ADO airport shuttle bus to Cancun airport. The trip takes 1 hour and costs MX$90.