@ Elodie (and others):

If you ordered a digital download, your initial e-mail with the confirmation (not the order confirmation, but the one with your preorder code) should have the digital rights number.

If you didn't order digital, but ordered physical copies of anything, your digital rights number will be in the packaging of your game.

If you can no longer find your number, first thing you need to do is call/e-mail SE and/or Digital River to see if you can get another copy. You're most likely going to need your order number for this.