In fact, they describe two set-ups; one cheap and one less so. The cheap setup requires GarageBand, the Rock Band USB drum kit and either the GamePad Companion, JunXion (trial) or the Xbox 360 Controller Driver if you've got the Xbox kit.
The pricier rig requires a paid version of JunXion ($95). After some simple configuration, you'll have a fully functional electronic drum kit with virtually no lag. Very cool indeed.
If you set this up, let us know how it goes.