WHY did this game get badges?
Seriously, this is ridiculous. I RARELY get the game to work right in the first place, and when I do, the lag combined with the horrible accuracy make it near impossible to do a damned thing.
I had this problem too, though I did not check the walk-through for it.
If it helps, in the same map that leads to the secret area for the lake area, there is a chest under the menu button, which was the one I was missing.
Zombies with pistols was just painful, zombies with machine guns was like "OMG, WHYYY?!", And now zombies with ROCKET LAUNCHERS?! There are no words to describe the ridiculousness.
Mind you, I like the game. It just seems to me that it's a bit over-kill to arm them when they already out-number you by the thousands.