Your objects, the ‘keys’ of your PiFace touch keyboard, are connected to the inputs of PiFace Digital. When you touch an object you complete the circuit. This happens because your body and the object are slightly conductive and allow a small electric current to flow. Because the current flowing through your body is so small you need to add another component to the circuit so the PiFace can detect it..
Connect a wire to the Ground pin (0V). You will need to hold this at all times when playing the instrument.
For each key, attach a wire from your object to the input terminal.
Now connect the input terminal to a 33Mohm resistor using a different piece of wire.
Connect the other side of the resistor to the 5V terminal (next to the relays).