Pretty sure you have to wait for the 29th, after having completed the mini games, to get codes to redeem on your versions online store.
Maybe the package itself will have a code to make sure people actually stuck with the pre-order, and dlc stuff isn't being handed out to non-pre-orders for free.
Not the code for the actual in-game DLC. The code for the Road Trip itself. The initial code that is supposed to be delivered by email.
It's essentially a code that tells the site "hey I preordered from amazon, here's my code to prove it."
You put that in, do the stuff, then on the 29th the site gives you the codes for the in-game items.