I just got this game as a gift bought from a retail store. I installed it and know that it is also on Steam, however the code I have will not work with steam. After a little digging I found a list of games that can be bought retail and then activated. Will this game ever be added to that list so that I can add it to my library?
As far as I know games which have a completely retail version cannot have their retail version serials used on Steam for activation.
Some games however MAY be sold with a Steam key which you can use, or give to someone else. Such as The Binding Of Isaac: Most Unholy Edition. Amnesia is not one that does this as far as I know. The serial you get however is used either when you first run or install the game, can't remember entirely
Sounds like you were given a Non-Steam version of Amnesia:TDD. This code allows you to download it but it is not the Steam version. This means that you can't track time played when you add the non-steam game to your library and it also won't show in your public games list.