Sudoku-Killer Manual:

In play mode, which is the default mode when entering, there are six different buttons enabling user interaction with the sudoku grid:
In solve mode the user can type in a Sudoku riddle into an empty grid to make the sudoku-killer solve the riddle. There are two options available in this mode:
Selecting a number is fairly easy and very convenient. Players just have to click on a field in order to open a digit menu for this field. To provide better overview and prevent annoying mistakes that can destroy some of the fun while solving a Sudoku, obviously impossible digits are not allowed to place within the grid. They are blocked, indicated by a more colorful and transparent background in the digit menu. Moving the mouse out of the digit menu makes it disappear again. Single digits can be erased by clicking the corresponding field.