Value of checked element in form

I'm helping out a teacher who wants to make web-based greeting cards for our school. My problem is that when I to get the value of the checked radio buttons in the form, it's always undefined. My form name is card, and I have an element named MIDI. I tried:
var MIDI = document.card.MIDI.value; but that didn't work. I checked my reference book, and it said that was how to do it. You can see whole script at http://www.geocities.com/thscards/. Any help would be greatly appreciated.

Thanks,
James