Hi,
Mach4 has 12 axes, six (X,Y,Z,A,B,C) are coordinated and six OB(Out-of-Band uncoordinated) axes, OB1 thru OB6. You might consider that Mach has one other motor output, namely a PWM output. The PWM output is usually used
for a spindle, say an analog controlled VFD or similar.

If you wish to use Step/Dir for a spindle then you have to use one of the OB axes. You might decide for instance that you wish to use OB1 as your spindle.

The first pic is of the Mach4 Control plugin, Axis Mapping Tab...where OB1 is assigned to Motor5. You could now set the properties of the motor.., like the pulses per rev etc in the Mach4 Control Motor5 Tab.
The second pic is the Mach4 Control Spindle Tab where you assign OB1 as your Step/Dir spindle axis. The Last pic is of the ESS plugin Spindle tab. Note you will need to assign ports and pins for the spindle servo
per normal in addition to this tab.

Craig