Heya.. the DLC isnt actually a DLC as such..If you have the code, open up Steam, in the bottom left is a "+ Add a Game..." link,. click that, then "Activate a product on Steam", then enter your code.
Hey Dark,
There isn't any issue with entering the code, the trouble is that Bethesda, Microsoft or whoever have not yet activated the codes for Australian players.
So here we sit on day 3 after launch not being able to use the Pre-Order DLC codes that came with the game because they are still 'inactive'