First, it's a 10mm Allen wrench you need, to access that double ended screw from the INSIDE of the reciever, and second, it's reverse threaded, so righty tighty will get you all screwed up. Screw left, and the stock will slowly come away from the reciever, but you MUST remove the screw inside the pistol grip first.
BTW, if you are installing the traditional folding stock, get BIG honkin' screwdriver, BIG sucker, and tighten that bad boy DOWN hard! Once that's done, it will work perfectly, lots o' fun!