You can try the C-Button trick. Switch your MAKERbuino off. Press and hold down the C-Button. While holding the C-Button switch your MAKERBuino back on. Hold down the C-Button for another ~3 seconds, then release it. The screen should stay blank for 20-30 seconds. You won’t see anything but the MAKERbuino is flashing the LOADER.HEX. After the HEX-file has been flashed you should see the known game selection screen.
This is simply a matter of turning your device off while something is loading and there is no need for testing the bootloader, just do the hard reset (it was also mentioned at the end of 4th chapter in the build guide: