My build (now with secondary shoulder buttons!)

[Flavor]

So, how are you editing/building that code?

Did you edit it and then
dkms build
dkms install
it?

You would probably want to increment the version number in the dkms.conf file (and also /usr/src/mk_arcade_joystick_rpi-0.1.5.5/ directory name) before doing the mods.

Then, after installing it, you could reboot (or modprobe remove/install) and then run
jstest /dev/input/js0

That's the best way to test your code, I think.

Also, what do you have in /etc/modprobe.d/mk_arcade_joystick.conf