How one uses level sensor?
It seems to behave as a Switch. One side can be connected to 5V(logic High), and other as an input to the Digital Pin of your Arduino.
Also remember it says its a NC so the input to the micro will be HIGH by default, if the level is reached it opens and would give a Logic LOW at the input.

Also remember to connect a resistor, or the best way to use it would be a connecting it via a Pull up resistor. Something like this: http://www.cooking-hacks.com/skin/frontend/default/cooking/images/catalog/documentation/tutorial_squidbee_y_squidbee_gateway/analog_sensor_1.png   but digital in your case.

EDIT: Please also read what Dzarda has added in the comments.