To add your requests to your reservation, you'll need to call 407-WDISNEY.
Generally, your chances of getting your request are better if you don't get too specific with locations. In other words, if you say you want a certain building, and they're not able to find a room for you in that building, they don't know why you wanted to be there, and they'll put you anywhere.
But, if you say 'close to main building', then even if the very closest building is not available, they can still get you as close as possible.
Or, you can phrase it this way: 'building C or close to main bldg'