Problem on updating mega 2560 ADK firmware

I have tried to flash the demokit.pde firmware on my mega 2560 adk board, however I found that the board can not run my program which downloaded from Arduino software. So I want to flash back the original firmware. But the problem is that I don't know how to change my board into DFU mode. Is there anyone one can help me?? =( thanks a lot!!!

I am not sure if I got this right. Have you re-uploaded the boot loader again? because As I think when you update the fireware, you need to re-upload the boot loader from Arduino IDE.

I hope this helps you.