It's simple. Fly into Cancun rent a car. The drive Cancun-Xcalak can take 5 hours on largely well maintained roads. In order to avoid driving in the dark, begin your trip before 1:00 pm. If that's not feasible you have two options. Spend an extra night in Cancun. Or, divide your trip. Consider driving an hour from Cancun to Tulum and spend the night. This second option provides you with plenty of time to visit Tulum ruins before completing the trip to Xcalak. Click here for some recommendations on places to stay in Cancun or Tulum.
Hop onto 307 South from the airport. Drive south for approximately 3.5 hours. Through Playa del Carmen take the over passes by driving in the middle lanes. Continue south on 307. About 20 minutes south of Tulum is the archeological site of Muyil. It is a much smaller site than Tulum, Coba or Chichinitza but has a pretty large pyramid and many buildings that have yet to be excavated. The beauty of it is you may only see another couple of people on the whole site. You can pay a guide to walk you through the site.
Continue on through Philipe Carillo Puerto. It's probably a good idea to get gas there. They will always have it. [UPDATE Oct 4, 2016] There is a new toll road that completely bypasses Filipe Carillo Puerto. Highly recommend taking it. Just follow the signs for Chetumal Cuota or Toll road to Chetumal. It will swing you to the west of town, through a toll gate, then put you back on 307 south bound past the town and the traffic.
Continue south on 307, watch for topes in Hu-May and Andres Quintana Roo, until you get to the town of Limones. At the south end of town, turn left at the sign for Mahahual/Xcalak. Follow the road for 51 KM. There you will hit a tope, make the turn to Xcalak. If you miss the turn and see the Pemex Gas station, stop and get gas then turn around and make the turn to Xcalak. The road narrows significantly at this point, as the jungle encroaches on the road. There are a couple of tunnels where you will be driving under a roof of trees. Drive carefully through this section as there are pedestrians, bicyclists, and wildlife that will be on the road and there are a lot of blind curves. This is the section that you really don't want to be driving at night. You will hit a T junction, with a lot of signs for Xcalak hotels, turn left. Follow the road until you run out of pavement. You are now in Xcalak.
The easiest way to get to the beach road, is when you hit the Xcalak soccer field, you will turn left then right, then go straight until you hit the beach, then make a left at the water’s edge. Go past the town pier, the road will turn left, then turn right and you will be on the bridge. Cross the bridge, then up the beach road 7.8 KM and you will find Acocote Eco-Inn. Parking is on the left side of the road, Hotel on the ocean side.
(Coordinates: 18°20'31.9"N 87°48'41.7"W)