I cannot find variant.cpp/usbcore.cpp

[luc-github]

Unlike 1.5.8, the 1.6.9 or up, doe not have DUE board support by default, you need to install it using board manager, check the process here : https://www.arduino.cc/en/Guide/Cores or in FAQ#166
Once DUE is installed, the files are located in your home directory in hidden folder : AppData

For example for the 1.6.8 Due module, it should be like :
C:\Users\luc\AppData\local\Arduino15\packages\arduino\hardware\sam\1.6.8 under windows,
/Users/luc/Library/Arduino15/packages/arduino/hardware/sam/1.6.8 under OSX.

path vary according version

The files which need to be modified (variant.cpp and usbcore.cpp ) are there.
EDIT: under windows now location is : C:\Users\user\Documents\ArduinoData\packages\arduino\hardware\sam\1.6.8\variants\arduino_due_x
and C:\Users\user\Documents\ArduinoData\packages\arduino\hardware\sam\1.6.8\cores\arduino\USB

[luc-github]

Another way to find them on all OS:
1 open IDE
2 go to >files>preferences
3 the path is wrote on bottom of dialog box
4 click on [...]preference.txt, it will open location to start find files

[luc-github]

for IDE 1.6+ Files are NOT in:

C:\Program Files (x86)\Arduino\hardware\arduino

[luc-github]

Under windows terminal, do a :

%USERPROFILE%\AppData\Local\Arduino15\packages\arduino\hardware\sam\1.6.8\variants\arduino_due_x

if you do not have something like :

or if variant.cpp size is not 28 911 you did not updated properly

[luc-github]

if you have different folders version under %USERPROFILE%\AppData\Local\Arduino15\packages\arduino\hardware\sam
it is wrong !!!
Delete all and redo board installation of Due module