Below, there are a number of options to get from the Cancun Airport to Isla Mujeres.
You can always take a taxi from the airport and there will be many willing drivers outside of the airport. This can sometimes be overwhelming with everyone wanting to help you at once. Their prices can vary considerably, but offer the convenience of just getting in and going. Always ask the price, before you get in the cab!
A very economical way of getting to the island is to take the ADO bus. This is a bus transfer from the Cancun airport, to the bus station in down town Cancun. (It was originally made for airport employees to get to and from the downtown for a reasonable price. Now it is quite popular among budget travelers.) Tickets are just 35 pesos a person and leave every half hour. Once at the bus station, it's a very reasonable cab fair to go to Puerto Juarez. It's a couple transfers to get to the ferry, but this is a very economical way of getting to the island.
Once at Puerto Juarez, buy a one way ticket to Isla Mujeres. There are departures from Cancun to Isla Mujeres every half-hour (on the half-hour) from 5:00AM to 9:30PM, then ferries at 10:30PM & 11:30PM. Once on Isla, the taxi stand is to your left or there are Maleteros (a friendly local with a tricylce to carry your bags. They'll lead the way with your luggage for a tip.) You can tell either to go to Hotel Las Palmas or the 'Mercado Municipal'.
If you're keen to walk to us, from the Ultramar Ferry: walk across the raised crosswalk (that goes between the Senor Frogs store and the 7/11) and continue down that street. It will open into the Zocalo (town square) On your left there will be 'the Express' this is the main grocery store. Turn left down that street after the groc store (the hospital will be on your right). We are about three blocks down, on this street (Guerrero) on the left hand side. You should be able to see our sign from about a block back and we're a yellow building. If you do happen to take a wrong turn along the way, ask anyone where the 'Mercado Municipal' is and we're right across from there.