Agree you should call MS ASAP to get the request on your reservation, and go to the front desk upon arriving at the resort - as early as you can. In the future, I'd recommend you contact MS when you make the reservation if you have a need such as this for medical reasons. It will improve the chances of the scheduler to get you what you need.
For as much as we love OKW, not including elevators in most of their buildings was a significant oversight and mistake. As a secondary request, you might ask for a building with an elevator. A few have them.