So why not just give the official site?
http://www.parsecproductions.net/slender/That fan site was established when the game first came out. Since it exploded faster than anyone expected, Parsec had constant problems with bandwidth overflowing, which was why people found it easier to host it there. But his site works now, has most recent downloads, etc.
Edit: Checking the Multiplayer video, your links are all wrong. The download link is for The Eight Pages. The video you linked is a Garry's Mod/general Source gamemode people made based off Slenderman.