The central bus station is situated on Fifth Avenue close to the Main Plaza and ferry pier. It's here you can catch frequent buses to the airport and to Cancun. Regular buses also depart here to many of the nearby tourist attractions such as Tulum, Chichen Itza and Coba. These can provide a cheaper alternative to going on an organised tour.
It's outside, just walk to your right once exiting the main door. I think you can purchase the bus ticket at the booth inside the airport (there's a stand there). No, you don't need to buy the tickets before you visit Mex.
There's a lot more information about transportation between Cancun Airport and Playa del Carmen: http://www.virtualtourist.com/travel/North_America/Mexico/Estado_de_Quintana_Roo/Playa_del_Carmen-936544/Transportation-Playa_del_Carmen-R-1.html
Have a happy trip!
If you only have a couple of people travelling together then it is less expensive to take the bus rather than a taxi/shuttle to Playa del Carmen from the Cancun airport. The bus runs every hour and you can catch it just outside of the airport. $7 USD each way and you can pay the bus driver.
Make sure you get your ticket the day before going to the airport. The first bus leaves at 8:00 and it will take you there in about 50 minutes, so you have plenty of time to catch any flight with two hours in advance.
Althought I didn't have any issues I was advised to get the ticket the day before by the people in the terminal.
I briefly thought about using the public bus to get from the airport to Playa del Carmen as the cost was much less ($71 vs. $24 for two round trip) but I'm glad that we didn't. We would have just missed a bus and would have had to wait for 1 hour 45 minutes for the next bus. When you are only there for 4 days, and hour and 45 minutes is a long time to waste.
The attached website had ticket prices and schedules
Buses are quite frequent to and from Playa del Carmen, and they are also quite cheap. They come in all ranges from deluxe buses to small, bumpy buses. It's just one hour on the bus to Cancun in the north, one hour to Tulum in the south, about four hours to the border between Mexico and Belize, three hours to Valladolid and five hours to Mérida.
We took the Rivera Bus from the airport to Playa Del Carmen. Busses are clean and air-conditioned. It's about 7 dollars a person! The bus station in PDC is centrally located at 5th and Calle 2.
The only issue I had was when we finally arrived at the airport all I wanted to do was get to PDC, check into our condo and chill. Taking the bus will prolong the arrival to your final destination. But if your looking for cheap transportation and do not mind the wait I recommend it!
There are plenty of buses that head to Valladolid, Cancun, Tulum, Coba, and many other places in the Yucatan peninsula. There are two bus stations: one for immediate cities/towns around the Mayan Riviera coast, and the other for inland destinations such as Valladolid.
Although you can choose between car rental, taxi, shuttle services, or airplanes, the cheapest way around the Playa Riviera or the Yucatan is to travel by bus. The buses run frequently and the PDC bus station is conveniently placed right by the hotels in town. It's at Portillo Juarez and 5th Ave. Buses go to Cancun, Tulum, Chetumal, Puerto Morelos, and Valladolid along with many other towns.