I think you'll find all the details on a site I set up earlier this year. I try to keep the information up to date, and have included a link to a map of Playa that shows the location of the 2 bus terminals.http://belizebus.wordpress.com
Even though the bus to Chetumal departs from the "old" terminal (not the new one at which you arrive), you can purchase the ticket at the new terminal,so buy it when you arrive; and I've found that the "new" terminal staff speak English (sometimes).
In Chetumal, I like to get the Premier bus (airconditioned) from the ADO terminal, but there are only 3 per day. If the Premier bus doesn't fit your schedule, there are frequent departures from the New Market (Nuevo Mercado) which is 1 or 2 km from the ADO terminal. These are old school buses. All the details are on my site.
Let me know if you have any more questions.