To use the sink in the bathroom or kitchen, you need to make sure you have water left in the freshwater tank. The main panel will display roughly how much water is left (the blue bar):
Turn on the water pump by pressing the water symbol on the main panel and then pressing the button that reads WATER PUMP:
It should illuminate orange and you should hear the water pump come on. Now you can use the sink like any normal sink. Remember to turn off the water pump after you are done using the sink!
To use the fold down sink in the bathroom the same applies. Simply lower the sink and turn out the faucet:
If you ever notice no water coming out of the bathroom sink, even though the water pump is on, turn the silver knob in the bathroom to switch water pressure from the shower to the sink.
Watch this video for a more detailed explanation: