Wait, did you redeem the code through Origin? Isn't that the way of losing the key? I've read it a few pages back, and even in the Amazon description page it's stated:
"The DLC Registration Code must be activated from within the game and requires a BioWare account. From the title screen select "Downloadable Content".
Only the base game code will activate through Origin."
Take a look at this post: HERE
Oh gee, that may be what happened to me. I'm not 100% certain what should show up on my Bioware page if the DLC activated correctly. There's a slew of items there. I'm not going to install the game right now, so I might have to look into it at a later time when I can check in-game.
Edit: this is the Dragon Age DLC on my Bioware page.
Edit x2 I experimented with the Mass Effect 3 DLC Motoki posted, and activating it in Origin did result in it appearing on my Bioware page.