The 12864 LCD is completely passive, the display must be driven by the (properly configured) firmware in the Mega2560...
Ok, so you need to:
Ok, so you need to:
- download Marlin, install it in your Arduino sketchbook
- modify Marlin's file Configuration.h, at least you need to set MOTHERBOARD to 33 and uncomment the line defining REPRAP_DISCOUNT_FULL_GRAPHIC_SMART_CONTROLLER (not REPRAP_DISCOUNT_SMART_CONTROLLER, that would be for the non-graphic 2004 LCD)
- download the libraries needed for your LCD into sketchbook/libraries, [code.google.com] + maybe some others...
- compile and upload Marlin to your Mega2560 (resp. correct any problems that the Arduino compiler might complain about ... until success)