The MPCNC software consists of three parts:
1. Software to create toolpaths as gcode files.
ESTLCAM does this but is Windows based so you would need to run it in a VM under Linux or OSX.
Have you looked at Autodesk Fusion 360 as an alternative? It has non-Windows options.
2. A gcode “player” to send the commands to the Marlin (or other) firmware on the Arduino microcontroller.
Repetier-Host is popular for this and it has Windows, Linux, and OSX versions.
3. A way to load the Marlin firmware onto the Arduino.
The Arduino IDE does this and is available on Windows, Linux, and OSX.
What I do:
I run ESTLCAM in a Windows VM. I use its built-in Arduino UNO (not Mega) firmware, firmware downloader, and gcode player. That way, I only need one piece of software (albeit on Windows.)