ThomsS ,to find your hex file:
1) In the Arduino IDE, menu File/Preferences check "show verbose output during compilation"
2) Build the firmware by clicking on the Verify icon
3) the last line of the console window will show you where is located the Marlin.cpp.hex file, which is what you are looking for)
Regards,
JPG (who had to recreate an account... because i forgot my pwd, and changed my email adress in between...)