Stepper frequency will be the result of the steps per unit, and the feedrate.
It's been a while since I've used Mach 3, but I don't think you even set the maximum frequency, it just relies on the max feedrate and steps/unit.
Also CS-Labs may move some of the settings to their own plugin.

Something to be aware off, is the older Leadshine drivers need longer than normal direction setup timing, but I'm not sure about the newer EMS drives. It was certainly the case with the older EM drives.