you do have to put the rewards on the rewards card first (you can get them at the parks if needed)...
when we went in January (stayed at the Poly) they made me put a real CC on the key, then the last night i just went down and paid it off with my rewards card... they had already charged my visa by the time i got down there, but she was really nice and put the money back on the visa and used the rewards card...
Just a friendly reminder.. you can't use your key for some tips in downtown disney.. didn't think much of it til we were at wolfgangs and she said i couldn't use it, had to scrounge up cash!