to the DIS! The best way to make a room request is to request what you would like at the time of reservation, or contact WDW (or your
travel agent) and have the request noted if you have already booked. CSR used to look at faxes, but it seems that the resorts don't pay attention to faxes anymore. There are some limited requests, like ground floor or near transportation that you can do during online check-in, but you can't request a building or section.
CSR offers preferred rooms if you want to be in casitas 1, 2 or 3 - that is a booking category, not a request. Otherwise, it is best to keep requests simple and limited - instead of requesting a building, request a section (like cabanas or casitas or ranchos) or a general location (close to Dig Site, upper floor, or close to main building). List requests in order of importance and limit requests to one or two things to improve your chances.
Enjoy your trip!