This thing requires that you run Chrome 14 or newer. I recommend a Canary build. (just run an interweb search for "Google Chrome Canary" and you'll find the right thing)
Oh also, point the browser to about:flags and enable "Native Client."
I've personally tested it on OSX and Windows.
It takes a little white to get started because it requires downloading about 6.5 megabytes of stuff and doesn't yet offer any kind of visual feedback to that effect.