If you want to read the AN0 or AN1 ADC input you would need to activate the Analog Input feature. You can purchase the activation key from the website.
The HiCON plugin diagnostic/status window displays a readout of the ADC input from 0-3300mV. You can also add a DRO using the screen editor and read the ADC registers using code similar to below:
local inst = mc.mcGetInstance() --Get the instance of Mach4
local hReg = mc.mcRegGetHandle(inst, 'HiCON/VADC_0');
local ADCCount = mc.mcRegGetValue(hReg);
scr.SetProperty("droADC0", "Value", tostring(ADCCount));