Your water tank will either be under the rear seat or mounted underneath the van itself. The underslung water tank has a screw-off cap that can be opened to completely drain the tank. If under the bed I believe they can be disconnected and removed to wash out. I have an underslung water tank and tend to use the sink tap to drain down the tank as far as possible.
The control panel has a master power switch on the left and two arrow keys to cycle through displays showing time, leisure battery voltage etc. To the right of the LCD display there will be the return/arrow button and switches to turn on the water pump, wardrobe and cupboard lights and van downlights.
If you press the water pump switch and turn the pump on you'll see the notification in your photo. You'll also likely hear the pump turn on. I don't believe the exclamation mark in a triangle is anything to worry about and just denotes a notification.
If there's water in the tank and the pump is on lifting the tap handle should cause water to flow. If it doesn't then it's likely either their is an airlock in the pump or the filter on the tap is blocked. Try unscrewing the very end of the tap and checking for any debris. If the tap is clear then you could try switching on the pump, opening the tap and trying to suck water through the tap to clear the airlock.
Hope that helps.
