Is there a way to unlock my AMD phenom2 555BE using a asus M4N68T-M motherboard.
check in its bios if ACC feature is available
ECC is not the feature you are looking for. Don't change the wrong things without knowing what they do. Worst case scenario, changing the wrong settings could fry your motherboard.
You can't unlock your processor with a Nvidia chipset based borad. ACC or Asvance Clock Calibration is a function available only AMD based chipset, available in AMD 785, 790 and 8XX chipset based motherboards.