This is from the mach3 yahoo group,

Config> ports and Pins> motor output> enable the spindle and enter the pin
numbers for the indexer step and direction pins.
Don't forget to click on apply.
Next, click on Config> ports and Pins> Spindle set up> put a check mark in"Use
Spindle Motor output" and another check mark in step and direction.

Last go to
> config> pulley set up
and provide the necessary info.

Then in your G code you will need the M3 command and the S value.
Such as S 200 for 200 RPM