Okay... I'm replying to my own question. Once I purchased it on Amazon, I tried to copy and past the code I was given directly into Steam. It did not work. Once I went through the formality of the Amazon download I was instructed to use the code with Steam and then it did work. Of course the DLC has not yet been relased and should download directly from Steam when it does, but apparently you need to use the Amazon download first in order to unlock the code for use with Steam.