that's weird, it does work, so that isn't the problem. It looks exactly the same as mine, however I think I've figured out what it is.
Just doing some reading, it seems that Xcode command line tools may be needed for the "purge" command to function, so I think you'll need to install the command line tools (sorry).
As you are running Snow Leopard, there is no official direct download for the command line tools, and I don't particularly want to link you to an unofficial one which may not work (it's here anyway though if you want to try).
Depending on your version of Snow Leopard (if it has the Mac App Store or not), you could download it from there, or use the aforementioned link that I don't particularly want to recommend as I haven't tried it, and don't want to muck up your mac if it doesn't work :/
P.S. It appears that apple are actually using the installer linked above now as the official one (https://medium.com/kr-projects/6e54e8c50dc8 (scroll down to the heading "Apple's Interest"))
Apologies for the slight rambly nature as I have just been writing this and researching at the same time