The numpad? I'm not quite sure what you mean. If you mean the number keyboard, then just tap the 123, like you do on a standard iOS keyboard. If you are referring to the little keypad looking thing on the right of the purple strip, it is not a numpad. It allows you to call up the basic operators that you see already there in your image. If you start typing, the purple strip will make suggestions. The little keypad allows you to mode back to the basic operators.