We've always taken the bus from CUN to Playa Del Carmen; it's about $11 per person, takes about an hour, and is very simple. To catch the ADO bus from CUN, exit the airport and turn to the right; you'll see them all parked there. You can buy tickets at the bus or you can get them from the ticket booth in the airport right before you exit.
At the bus station in Playa, exit through the doors and walk toward the beach (very easy to see which direction to go). The ferry terminal is a short walk south along the waterfront; you can almost see the ferry terminal from the bus station.
Ferry tickets are about $13 pp; as was mentioned above, there are two or three different ferry companies and they're essentially all the same (price, service, etc.) Just make sure you get in the right line for the ferry you paid for. If you're not sure, ask.
As an alternative, there are a couple airlines that fly small planes from CUN to CZM. One is Mayair, and I'm blanking on the name of the other. I've used these a couple times... a quick 20 minute flight from CUN to CZM or the other direction. Cost is around $60... a little more expensive than taking the bus and ferry, but a lot quicker and much more convenient. A bit of internet searching should let you find the options.
Hope this helps!