Guys,
 
I checked with the event host organization (AMIPCI).
 
Here's what we must do:
 
Once you have your baggage and exit customs, you will see
some booths that are identified as "Servicio de Taxi autorizado".
You tell them your destination, pre-pay the trip and walk outside.
You then look for the taxi rank (it is identified and the cabs are
painted yellow and white, with graphics depicting a plane on
the doors.
 
You should be careful, when pre-paying the fare, to specify that
you want to go to the "Hotel Sheraton Centro Historico, frente a
la Alameda Central" (because there are three Sheratons in the city).
 
Hope this helps, regards
 
Tony Harris