Hi Sylvain,
READ ERROR message might appear when your wiring is incorrect.
Can you try running i2c scan to see if your device is connected correctly and check what address it uses?
http://playground.arduino.cc/Main/I2cScanner
To change i2c address, see: ArduinoMotionSensorExample/mpu.cpp at master · rpicopter/ArduinoMotionSensorExample · GitHub
Value 0 = 0x68, Value 1= 0x69
If this does not help please specify #define MPU_DEBUG in mpu.h and recompile. Post the output (MPU Init value).