Amnesia: The Dark Decent Custom Story HELP!

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I know that a lot of people ask this question, so I'm sorry for asking it again.

I have finished the game.. I think. I thought that when you finished the game, that one of the monsters in the game showed up on the home screen every time you enter.. but maybe not. I don't know.

My main question is, how do you download and put custom stories into the game? I bought the game from E-bay and not off of steam. Does that have anything to do with why I can't play them? I've been on YouTube a few times to see how other people have done it and I have followed every step exactly as they have shown, and it wont work..
Any help would be greatly appreciated. Thanks!