The last firmware (Atemega32U4) is http://SimHQ.com/forum/ubbthreads.php/topics/4028164/Re:_MMJoy_-_Build_your_own_USB#Post4028164

You can load the firmware with USB cable (a bit tricky) or use USBASP cable (V 2.0).

https://code.google.com/p/mmjoy/wiki/UpdateFirmware_eng