I've connected the stepper motors to the shield. Uploaded the sketch to the arduino. When I give the command to 'move pen to point' the motors start turning, but they never seem to stop. Even when I reset, just set a small movement, it keeps turning forever.

I'm using the files from the bundled software Polargraph 1.10.6.

I have tried the AFMotor_multistepper example sketch. One of the motors turn and reverses, the other just turns in one direction but with a slight jitter.

Any advice? If you need more information about my setup, please tell me.

I'm assuming you're using an Adafruit Motorshield v1. Yes AFMotor_multistepper waves one of the motors back and forth, and sets a really distant target for the other motor, so it should go on for ages. There's shouldn't be a slight jitter. Is there plenty of torque on that motor? I mean, can you stop it with your fingers or is is a good, steady power? I've had situations where I've wired the motor up wrong, and it'll turn, but there's no real power behind it.

Right, either way, if it can get stuck turning forever, then I think Kong is probably right. There is a lot more to running the machine than uploading the sketch and firing commands at it - which guide are you looking at to help you set up? The original instructable is still applicable (http://www.instructables.com/id/Polargraph-Drawing-Machine/step18/Finish-configuring-your-controller/).

The main thing I see not mentioned is "set home". Without setting home, then the behaviour you describe is expected I think. Until you tell it where the pen is (with set home), it defaults to some impossible position up in the sky somewhere, and so any "move to point" will naturally take an age.

Like Kong says, the debug console is your friend here, but also following the guide.

sn