How do I send the PWM signal from Mach3 to Pokeys 57E?  I can scale one of the 57E PWM outputs to 10V easily enough, but I can't figure out how to get the Mach speed signal into the 57E.

It should be really simple - open the PoKeys plugin configuration, switch to PWM tab in the settings, enter the PWM period in microseconds, then select what you want to do with each PWM channel. Simplest is to use the 'Map to DRO in %', which will use the selected DRO's value (multiplied with the optional multiplier setting) as the channels duty cycle.