Actually, Troy, Tim, and Joseph (The creators of Marble Hornets) have funded their season 3 kickstarter which, as a stretch goal that has been funded, they are going to film excerpts from the actual Marble Hornets film that Alex Kralie was making! So we will be seeing more of Alex's cheesy filming in the future!
1) It's this exact file: http://www.freesound.org/people/jobro/sounds/46415/
2) It's a combination of a cluster chord on a piano (essentially, slam the keys with open hands to hit as many notes as possible), with some strings behind it; probably violins, but made slightly quieter than the piano. They are discordant, and performing a small glissando up to a short note.
3) Here's my best guess as to how you might notate it: http://i.imgur.com/us0mR.png
There was/is an official page for it, though I don’t remember the URL. You could search “Slender Official” or something, maybe that way it will appear.
Edit: here it is http://www.parsecproductions.net/slender/
Ok, now I'm starting to get a little bit paranoid. My roomate has been coughing pretty heavily for the past 15 minutes.
Edit: I just sat down, and started using stumbleupon to dick around on the internet, and this is the first web page I came across. I'm screwed.
Here's a google doc with the game's setup.
The game ended up being way overpowered for evil. Although the slender man players didn't know what happened when they killed/visited players, the good players were too in the dark to figure out what was going on. All the pairs had revealed on day 1, which meant the slender man players could get double kills very quickly. The information that was revealed in the thread wasn't trusted as good information by others. Also, because of the "He only exists because you think of him. Try not to think of him." saying, there was almost a day of no posting because players thought by not posting (not thinking of him), they would prevent him from killing or something. After half the day was wasted, I finally informed them that wouldn't work.