It's not that hard.
If you're using steam the file path will be something like this:
C:\Program Files (x86)\Steam\steamapps\common\amnesia the dark descent
otherwise if you're using the retail distribution (if you bought the game anywhere other than steam) the default file path for the main folder is this:
C:\Program Files (x86)\Amnesia - The Dark Descent\redist
(I am of course assuming you're using Windows 7 or Vista. Otherwise the instal path will not have the (x86) in it)
In order to find the folder, open Computer or My Computer, depending on which version of windows you're running, and double click on Local Disk ( C: ) and just follow the file path from there.
Once you get to that folder, you should find it full of folders such as "billboards," "commentary," "config" etc. Be careful not to touch any of these files, as they can potentially mess up your instal of the game and you will have t re-instal it. Instead, find the folder named custom_stories, and you're done.
Hope that helps!