Hello. I was wondering what is a good, reliable place to download custom stories for this game? Also, how do you install or add one? Thank you.

A good start is Moddb. Take a look
http://www.moddb.com/games/amnesia-the-dark-descent/mods?sort=ranktoday-asc

One of the better CS is White Night
http://www.moddb.com/mods/white-night

Tutorial
http://www.moddb.com/mods/white-night/tutorials/how-to-install-total-conversions-how-to-run-them

For normal CS and not Total Conversions look here
http://www.youtube.com/watch?v=_pYpOQri3UM

Aside from White Night, any other recommendations?

So many. Looking through all the crap on moddb, it's hard to believe that there are actually a LOT of really good custom stories. Most of them are total conversion mods though. I'd recommend Amnesia Hill, Abduction, Tenebris Lake, and Disponentia (all of which are on moddb)

Holy crap. Did not know Amnesia generated that much interest. Most people I talk to in person are like "lolwutamnesia, why it not LoL, lol?"

'The Great Work' is one of, if not the best custom story out there, it's good enough to be official DLC.

The Four Horsemen :

http://www.moddb.com/mods/the-four-horsemen

Where do I install/extract the new files to? Sorry I am new to this, and thank you for your help :)

Amnesia\custom_stories

Why can´t I use the "Level Editor"?
It always says that the game crashed everytime I try to use it.
I´m guessing that the "Level Editor" usually is the program to use
if you want to create your own custom stories, no?

Amadeus - from my research one of the only games to have a rating of 9.0 and above is Amadeus at 9.6. The only other game I can find with a very high rating is White Night with 9.0.

PS: Just found 'Things in the Night' - it's rated as 9.2.