Hi,
sorry should explain how to use it.
One end, doesn't really matter which, goes into the output you want to test. The other end you hold on a 5V terminal. If the output
is low one LED will light up. If the output is high then neither will light up. If that's the case hold the floating end on a 0V terminal.
If the output is high a LED (opposite color) will light up.
Craig