#define MOTHERBOARD BOARD_MKS_13 should work...
really need to see the error....
My only guess is perhaps your using a old boards.txt file that is not designed for newer Arduino ide.. In which case just change the board type in Arduino to mega2560
really need to see the error....
My only guess is perhaps your using a old boards.txt file that is not designed for newer Arduino ide.. In which case just change the board type in Arduino to mega2560