Hi,
I'm hoping a mod will sticky this so that anyone asking the question can easily find it without posting or digging around.
I'm not asking this question, I'm answering it for anyone who wants to know:
If you bought Amnesia: The Dark Descent on Steam:
A patch came out recently. Update your game, then start it up like you would before. On the launcher, there is a new button called "Justine." Press this button to play the expansion.
If you bought Amnesia: The Dark Descent at retail:
The Justine expansion pack is
now available for all retail users! To get it, you must download the version 1.2 update from Frictional Games' update page, which can be found
here. Choose the option that is right for your system, download the package and install it, and then start the game as before. In the lower right corner of the launcher, there will be a new button called "Justine." Press this button to play the expansion.