To redeem this, you will need to log into Darksiders while connected online between 12 Oct - 22 Oct 2012 and the content will be sent to your Tome inbox. Go to the nearest tome to collect your items. Full details here.
A bit of a hassle if you ask me. Personally I prefer the DLC method. Just allow us to redeem a DLC code online or put the DLC up for free in the Playstation store for this period.. Oh well.. I guess THQ wanted to reward only the players who actually have the game.
I have this game but yet to start playing it. Will I be able to reach a tome at the start of the game or will I have to play quite a bit before I see my first tome?
Anyway it's free so I'm not gonna complain much. It's cool and I love free stuff. (Don't we all do?).
Edit: Apparently, it takes about 1 hour before you reach your first tome after starting the game. Just little info for those who have the game but have yet to start playing.
Source: Darksiders Official Site