We have a policy on this site NOT to post the 800 number for
DVC Member Services. The local number for MS is 1-407-566-3800 .
The person who you rented from should be the source for providing information about confirming your reservation (including providing the confirmation letter from Disney). That information would be appropriately found in the contract provided for your rental- along with the resort, room type, dates and deadlines for any payments agreed upon.
You can call the resort directly and see if your reservation is in their insternal system yet (DVC point reservations are not available with the Disney resrevation system- they are sent to the resort a short while before the check-in date).
I'd contact the person you are renting from - to provide you with information about checking your reservation and certainly to get the confirmation letter.
Have a great trip!