ok, I took a little look at the detail behind the shield.
First up, I would be surprised if the shield just needed power to start displaying info on the screen. From the schematic that is online (I can’t vouch for it’s correctness) the LCD control pins go to the waspmote connector only; unless you plug in to that, you’re not going to get any displayed text.
So when you power it up you said the piezo buzzer sounds; that means there’s radiation there, right? It’s directly connected to the output of the tube and counts pulses, so when the piezo buzzes an LED should also flash, and the interrupt pin should trigger. What do you see from your code?