Hey, you're not supposed to add the entire folder that the zip made... just the WifiOnOff app. Those other documents are the source code and you don't need those. Just drag WifiOnOff to your Applications folder and it will run fine. I got a decompression error as well and it still worked perfectly after manually decompressing it from the Finder.
Ohhh, I read the instructions on the site and thought they meant to transfer the entire folder.