Key will appear, LIKE MAGIC, on SteamGifts after you win.

Don't forget to claim the free DLC for Castaway Paradise:
FREE China Theme Pack
FREE Soccer Theme Pack
FREE Awesomenauts Theme Pack
FREE Villager Stevie
FREE Ranger Theme Pack
FREE Boutique Theme Pack
FREE Hospital Theme Pack
Version 3.0 Beta - Including Controller Support
(They're all on steam)