I too have tried to cobble this together with zero success..
The geiger tube side is fine and I've popped in a simple transistor circuit to register events by flashing an LED and clicking a piezo and waving it near radium dials on old clocks sends the thing haywire so no problems there, however the Arduino interface fails.
Earthing pin 2 registers a count, tapping away against earth registers multiple counts, but nothing I've tried beyond that works. I added multiple transistor switching circuit variants and even a pulse extender but unless pin 2 goes hard to ground there's no count.
I am a total newbie to Arduino so peering at the code leaves me bewildered however I see there's a few things in there that even to my novice eye make no sense or look weird.
I suspect we'd need a competent arduino coder to eyeball it and make a decent amendment to the code for us to use. I'd also prefer the thing accept a rising pulse rather than trying to use the pulse from the geiger tube to switch pin 2 to ground since that's a lot easier to work with
Basically whatever I do with the basic geiger tube circuit works, whatever cobbled together LED/clicker works whether connected via capacitors, resistors or transistors - but nothing whatsoever gets the Arduino to count