Simply unzip the downloaded package somewhere on your PC and run "quantoon.exe".
While some Windows versions allows it, running the file from inside the zip won't work, you have to extract the whole folder.
Best if you run it from your hard disk or an USB key; if ran from a CD it could lag a bit between longer scenes, as the CD spins down when idle and needs some time to spin back up.
On Linux you can succesfully run "quantoon.exe" under Wine, in some cases it runs better than on Windows.
Mac users (and actually everybody else) can open the file "quantoon.html" in any web browser that supports Adobe Flash.