You select the number of dice you require using the slider in the middle of the screen.
Below that are a row of buttons to modify the roll types.
The first button "-1 NoA!" removes a success for each 1 that you roll, and there are no "again" rolls.
The second button NoA! is the standard roll button. You get a success for any 8, 9 or 10, no rerolls and no penaties for 1's.