Use the arrow keys or select a square with the mouse to move the cursor from square to square.
Tab - moves to the next clue in the same direction.
Spacebar - Changes the direction of your typing.
Backspace/Delete - erases typed letters.
Click on one of the numbered boxes in the unfolded cube to change to a different side of the cube (or press numbers 1-6).

See in-game help for detailed instructions.