From the Gamepad manual entry:
[quote:3immz1dq]The Playstation 3 controller does not typically work without installing third party drivers since it is not officially supported. For this reason it is recommended to test with Google Chrome on Windows with an XBox 360 controller, and always support an alternative input method such as Keyboard input.
So you can get it to work, but it doesn't necessarily work out of the box like the Xbox 360 controller.