I am trying to use a stepper as a spindle, but can not get it to work. Any ideas?

What are you driving it from?  What do you mean by "it doesn't work"?  Mach assumes that the motor runs at a speed which can be set, for example by smoothing the PWM output to get a voltage for the motor drive.  It does not generate a stream of pulses at a variable frequency to drive a stepper.  IMHO a stepper is a very poor choice for a  spindle drive, why do you want to use one for that purpose?

Hi,
its been a whaile since I've used Mach3 but certainly Mach4 has the option of a step/direction spindle, and if memory serves I think
Mach3 does as well.

I do agree with the previous poster, steppers just plain run out of steam the faster they go. A step/direction servo is a good choice though.
I have a 1.8kW 3500rpm 6.2Nm (cont) Allen Bradley AC servo and drive as a spindle motor and it kicks arrsse!

Craig