Sorry but i've got a little to do so i have tried the circuit proposed by johnwasser only now.
Same result as Pieter_C. Only a bunch of 0...
So i' don't know why but i'm quite sure now that what we have in our hand is different from the piezo's shown on the Starter Kit page.
I've seen other image of the piezo and mine effectively seems a little different at sight.
I suppose that me and Pieter_C are just unlucky!
But thanks anyway!