Keeping score

I’m trying to make a little jepordy style game for my website and I want to be able to keep score on it. I found the tutorial offered here. I was able to get the downloadable source to work fine but unfortunately I can’t get mine to work. Everytime I click my button the dynamic text box that is supposed to display the score only displays the word “NaN”??? I don’t know what I’m doing wrong, does anyone have some advise?