When you launch the game it tells you to shift-tab for your serial numbers. DO that, and then enter them through the "redeem code" option in the game.
If you (or someone else playing your game) foolishly closed the small window with the keys in the past, they won't show up anymore. I'm sure if you muck around in in steam by right clicking on the Dragon age game, you should be able to find them again.
Oh, I forgot to mention that one of the keys is the actual CD-key. This one you need to enter as "registering the game" at the Bioware site, rather than "redeem code". Once you've redeemed your two codes and registered the game with the third code you should have all of the DLC available that comes with that version. The CD-key is different from the 2 redeemable keys, so you should have no problems detremining which is which.
open up your steam game list click on dragon age and select show CD key, the 2 top 12 number codes are what you use to redeem the DLC. click on redeem code ingame and it should take you to a web page where you enter the code, you may have to setup an EA/Bioware account before you enter the codes though.
1) you have to create a Dragon Age account and log in from inside the game 2) in steam, you can right click Dragon Age and view what they are calling "multiplayer keys" 3) copy the deluxe edition key, and register it from the EA Dragon Age account. 4) DLC should instantly move to the downloading tab and start coming down.
Took me forever to figure this out last weekend - they really should have a guide or something on how to tie to two together.