It's odd, but I don't have to enter any code. I just have to be logged in to the forum.
I click the Click Here link in the email. That takes me to the forum page for the specific download, though I'm not logged in. Once I log in to the forum, then the page reveals a link to download the .pdf:


That code you guys are talking about sounds like it provides access to a subscriber, but if you ever access the DLC page and don't see the file link, scroll up and see if you're logged in or not. Click to log in, and see if the link is now revealed. I bet it will be.