Go to
http://www.kd-dietz.com/ and download the machscreen software.
Install the software and open it.
Open your screen file (usually under mach3 main folder).
Found the Mist Button and click on it.
There will be a one more window for settings dro and buttons oem codes etc. Go to there and click on hotkey after press a button on your keyboard which button do you want to setup hotkey for mist.
Save screen and open mach3.