you dont include the < and > tags in the javascript code, I onlyt used them to denote variable values.
onChange="document.<eCart1_1_ATC_01>.<eCart1_1_Description_Add>.value=document.<eCart1_1_ATC_01>.<eCart1_1_Price_Add>[document.<eCart1_1_ATC_01>.<eCart1_1_Price_Add>.selectedIndex].text"
should be:
onChange="document.eCart1_1_ATC_01.eCart1_1_Description_Add.value=document.eCart1_1_ATC_01.eCart1_1_Price_Add[document.eCart1_1_ATC_01.eCart1_1_Price_Add.selectedIndex].text"