There are several acceptable variations on the address, but all that is really necessary to write on the envelope is two lines: Santa Claus, North Pole. The post office will forward it to the proper address.
Santa doesn't have a real address. If you want to send the letter off you could always write on the front of the letter - Santa Claus 25 Reindeer Lane, NorthPole 345083. Your post office will then receive the letter and send it to Organization Projects to help the needy during Christmas time.