Step one: Go to the Kraft web site and order the Kraft dance pad ($9.95 shipped). It's a full size dance pad, similar to the ones you've seen in game stores.
Step two: Get StepMania. This is an open source "Dance Dance Revolution" knockoff. It has loads of downloadable visuals and songs and such. It's really a top-notch piece of software.
That's it. The Kraft pad is intended to be used with Shockwave dance games on the Kraft website, but Windows thinks it's a standard USB joystick, so it'll work with pretty-much anything. I verified this myself (I'm the "John" mentioned on the StepMania website).
It's a standard USB joystick, so I presume it'll also work with the OSX or Linux version. I can't verify that, though.