You can fly directly to Cancun from most major airports or if you chose you can fly to Merida.
You can get a bus directly to Merida from Cancun. From the Cancun airport, either take the shuttle bus or taxi to the main bus terminal down town.

From here you can jump on either the ADO www.adogl.com.mx/en/index/htm or the UNO www.uno.com.mx/english/index These are luxury buses with fully reclining seats and free drinks. The cost is about $30, or £20 each way. The schedules are very regular.

Another option is to hire a car at the Cancun airport. We have used easyway and they are very good. http://www.easywayrentacar.com/index.php . Make sure you fill up with gas before the trip. From the Cancun airport, take the main toll road to Merida, then follow the signs for Progreso. From Progreso, follow the signs to Chelem. There is now a new bridge that takes you directly to Yucalpeten. Here, you will see a small police station on the round-about, or "glorietta." From here, you need to get onto the beach road. You will see a sign for the Si'an Kh'an Hotel. If you follow the sign, you will see a big pink building that is Casa Rosa. Total travel time is approximately 4 Hours.