From the page you linked to:
Quote:
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*This item may not be fully compatible with Dance Dance Revolution Mario Mix for the GameCube.
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I've got a whole pile of converters that didn't work, until I finally
found one that does. As stated in that post, it's the
Cube Joybox Pro, but it looks to be out of stock at P-A right now. They may get more soon, or you may be able to find them elsewhere. These adapters worked great on the GameCube, however, and specifically say on the back that they're DDR compatible. Every other one I tried either failed completely, or failed by not allowing you to press up and down or left and right simultaneously.
RedOctane Universal Adapter (PS1/2 to GC/XB/USB) 
