To begin choose game level (lame, beginner, advanced or wizard) and press New game button. You will see randomly generated puzzle. To put number in empty field, click on small green digit. Status box at the top shows moves remaining to game end. To undo last move, click on Back button.
Static puzzle numbers are black, placed by player are blue. Clicking on X removes them. Small number from 1 to 9 can be green, black or light grey - try to click several times on them with right mouse button. These colors don't have any meaning to program. They are only for player - you can check which numbers you plan to put in the field, and which not.