The digi display will only come on when the heater fires up and stays on.
Yes the switch for the ignition is on the bottom. Can't remember at the mo which way it is to turn on but you will soon know the right way as when the water flow is turned on the ingnition will start clicking to fire the heater up.
If it clicks and does not fire up make sure the gas is turned on, the water flow dial is turned all the way to minimum and the flow of water on the pump is set high enough to fire it up. You will soon know when it fires up as will hear it, will stop clicking and the digi display will turn on.