Use mouse or arrows of the keyboard to select a cell.
Use Numbers of the keyboard to fill in.
Use Del or Backspace to delete.
M - shows/hides help numbers .
B - shows/hides all help numbers.
А - Inverts help numbers.
Shift + 1/2/3/4/5/6/7/8/9 - Marks help number.

More help information and sudoku rules can be found here.