Under construction
If you have manually flashed your MCU using the programming pins on the MCU board, it is likely that your MCU does not contain a bootloader. The bootloader is required to allow flashing the firmware from the display unit. You'll have to manually reflash the bootloader using the same method again, either using the default arduino bootloader compatible with the ATMEGA2560, or flash this ReBrewie 4.0.0 firmware + bootloader image.