I'm trying to enter the code on the back of door number 1 into the Lego Star Wars site but it doesn't work. Does anyone know how / where to input the code?
By "the build instructions" you mean how to assemble the day one model? It's right there on the flap. I doubt there's anything more to be had.
I just found this website:
it has special stuff each day, the place you enter the code is the image of jango fett in the bottom middle.
The instructions to build it are on the back of the flap and to enter the code go to lego.com/starwars and go to fun and games or something. It mentions games. Anyways, there is a game called "Advent Calendar" and you click on a button torwards the bottom of the game screen (which you can make fullscreen) and enter the codes. They just unlock minifigures that you can be while playing the game.