Join the LoopMachine Facebook Group
The Wii Loop Machine 1 is open source and available for free download. It requires Mac OS X (10.4.8 or 10.4.9 recommended) and bluetooth. Unfortunately it does not work on 10.5 Leopard at the moment. No installation is necessary, just download, unzip, and play! The application contains directions and troubleshooting tips. Version 2 is currently being developed, and in all likelihood Version 1 will not be updated. 
Download the Wii Loop Machine source
The software was written in Max/MSP, a graphical programming interface by Cycling 74. You can download a 30-day demo to try it out.