The Stepper library is the wrong library for step/dir type of stepper drivers like the M556c driver. The Stepper library is written for drivers like the ULN3003 (unipolar) or the L29x drivers (bipolar). The M556c driver will not work with the Stepper library.
You should be using a library that is written for the step/dir drivers like the AccelStepper library or the MobaTools stepper library. I think MobaTools is easier to learn. There is documentation and examples to help to learn it.
Here is the manual for the M556c stepper driver. See page 3 for how to wire to a microcontroller. I would wire all of the - terminals to ground and the step, dir and enable terminals to outputs of the mystery Arduino board.