Thanks for this - alas my electronic skills are limited to assembling Veleman kits. I'm glad you are puzzled, I was too - I thought it was just me!
I am using a nominal 12vdc 500mA supply. With no water the output on pin 2 is high (+13v), with water, low (0v).
I don't yet have a motorised valve, I was assuming that once I got it to operate a relay I could do anything I wanted - actually for a start just to ring a bell.
The great advantage of this module should be that as it is contactless it won't suffer corrosion, or need special drillings - it can just be glued to the outside of the plastic tank.