DriverDiary was designed to also be used on a car computer with a small touchscreen.*
If DriverDiary is being used on a touch screen, go to the program options (Tools -> Options) and enable the option "DriverDiary is being used on a touchscreen" to enable the retractable number keypad.
You can enter numeric values on the fill up and maintenance screens by using the keypad.
*Fields requiring text input throughout DriverDiary require the use of a keyboard.