I just checked this to see how you can do it.
If you go to my reservations and enter you reservation # and birthday you will see the My Reservations screen. Click on Modify under My Reservation.
The Please Wait Mickey Boat Screen Comes up and then the screen to make changes to your reservation.
There is a box on the bottom right hand side that says Linked Reservations. If you click on that it gives you the option to link your reservation. You will need the reservation # of the cabin you want to link and the birthdate of the lead person on the reservation.
If it is already linked it should show after you hit the linked reservations #.