Start from left to right and use the up and down arrows to show a number from 0 - 9.
Once you have selected your number then click the number itself and the square will turn orange
Move to the middle box and again use the arrows to select the number to add and once chosen click the number itself
and the result will be shown in the box on the right
The sequence only works from left to right and you must click each box in turn
(Doesn't work in Opera or IE6).