Official post in the Steam forums:
"Amazon sales with no DLC
There have been some issues with Amazon customers not receiving the pre-order bonus DLC. Amazon actually chose not to participate in the pre-order bonus but our distributors Mastertronic have some codes left, so if you would like one then please contact
support@mastertronic.comPlease note, that this is ONLY available if you're able to show proof of purchase."
I sent an email with proof of purchase to Mastertronic and got the code by email within 10 mins.