Merida

By Air
Merida’s Manual Conscencio Rejon International Airport (IATA: MID), is located 7.5 km south of the city along Hwy 261 and offers services to all types of flights, private and commercial. There are daily flights out of the Airport to México City, Houston, Miami, Belize City, Havana and a few other cities in Mexico.

The following airlines serve Merida:
Aeromexico, Aeromexico Connect, Mexico City, La Havana, Miami, Monterrey & Veracruz
Interjet,  Mexico City.
Magni, Mexico City.
Mayair, Cancun, Cozumel, Villahermosa & Veracruz.
TropicAir,  Belize City.
United,  Houston.
Viva Aerobus, Guadalajara & Monterrey.
Volaris,  Mexico City, Guadalajara & Monterrey.

To get from the Airport (by ground transport) to your destination locally you have several options:
-You can take an authorized taxi cab or van, at the Airport for M$200-250. Buy the ticket in one of the booths at the exits of the airport. Likewise you can also walk out past the main parking lot in front of the building, along the access road out to the main road (Calle 23) to hail a passing taxi for half the price.

ADO operates regular buses from Merida airport to the main CAME bus station (Calle 70 entre 69 y 71, #555) or up to the Hotel Fiesta Americana (Calle 60 por Avenida Colón).
From the terminal building the bus stop is on the island by the second access road in front of the arrivals

There are more flights with additional tour operators (on charter airlines) and regularly scheduled airlines to Cancun (CUN) from Europe, North America, Central America, South America, Cuba, Russia, and additional cities within Mexico. With the extra competition between tour operators and airlines, it may be more economical to fly to Cancun and then drive or take the bus  to Merida.

There are a limited number of direct buses to Merida from the Cancun airport with ADO (Autobuses De Oriente) [10]. Otherwise one can take a more frequent bus to the downtown Cancun bus station from the airport (every :30min – 1hr) and transfer to another bus to Merida.

By bus
If coming from Cancun you can either choose to go to Merida’s main terminal (CAME) or to the “Hotel Fiesta Americana” which is a short taxi ride to El Centro.
Note 1: There is a toll highway (Hwy 180D) between Cancun and Merida that is in excellent condition. The ADO Platino and GL Buses do the 380 km trip in just about 3.5 hr; there is a non-toll road (Hwy 180) that parallels the highway that goes through 70+ hamlets and 40+ villages and is both slower and bumpier and, the second class buses go along this route.
Note 2: There are frequent buses between Cancun Bus Terminal and Merida CAME or Fiesta Americana and both destinations are about the same distance from Merida Centro; CAME is in a somewhat industrial area while Fiesta Americana Bus Terminal is right in the top sights area of Merida.
Note 3: There are a few direct buses from Cancun Airport to Merida alternatively you can get the free shuttle bus (every :30min) from the Cancun Airport to the downtown (Cancun) bus station and transfer to another bus to Merida.

Source/http://wikitravel.org