Oh neat!
So what does it do to your game? Does it overwrite it? Does it just toss an extra option into your list of options at the start of the game?
It overwrites it, so you should backup the original (as the README.txt included in the download says). The extra stage also has a mode selection based on which enemy you kill at the beginning of the stage.
I also forgot to mention that the mod is available through thcrap too, which is probably the best way of using it.