I haven't used L298Ns before so I can only guess, but I believe the control mechanism is the same as the ULN2003 - four wires that link to an H-bridge, pulsing in the certain order makes the world go round.
Internally, Polargraph uses AccelStepper for motor control, so you could try the example sketches that come with AccelStepper, and try to get them moving.
I notice in configuration.ino, that the pin sequence is odd:
AccelStepper motorA(8, 6,8,7,9);
6, 8, 7, 9. I don't understand why, and this might just be an artefact of how the person who wrote that up had their motors wired. I only accepted the pull request for those lines, I haven't actually used it!
Have a shot trying the examples that come with the accelstepper library, that'll let you troubleshoot your wire connections.
To reverse the motor direction you can just reverse your motor wiring. You can always do it in software, but there isn't an easy way like there is when using serial stepper drivers.