Dajax: having trouble with the simple form in dajaxproject.com?

Question

I'm trying to test a simple form that display options in a second combobox by selecting option in the first one. So I tried to use dajax and I started by testing a very simple form exists in http://www.dajaxproject.com/forms/ I do all what they said in the tutorial but I the function 'updatecombo' called in the first combobox do not work(the second combo doesn't show anything). I think this is because of not including dajax in my html file. Any suggestions?