Arduino classes, can't get property value
When you put the type there, you are creating new variables.  SO in your constructor:

 MKSStepperMotors(int dirPin, int stepPin, int enablePin) {

      int _dirPin = dirPin;
      int _stepPin = stepPin;
      int _enablePin = enablePin;


You start by creating three new variables with the same names as the member variables defined in the class.  Throughout the rest of the constructor those get used instead of the ones that are member variables.  

You gotta lose the "int" there.  

You also have another bug in there.  The constructor may run before the init function if one of these objects gets declared at global scope.  Things like digitalWite and pinMode aren't going to work yet in that case.  A constructor should just have code to initialize variables, it should never touch hardware.  The stuff that uses hardware should be in another member function (usually called begin or init) that you can call from setup.