I think both "on" and "with" are correct. But by using "on", I think the action of her entering the numbers is emphasized more. It's a very subtle difference, though.

Hi shigeki If you was to say 'A women is entering numbers with a keypad' it would basically mean that the women is accompanied by a keypad whilst entering numbers. It's like saying 'a women is writing on a table' or 'a women is writing with a table'.