Okay, I see. I have just successfully downloaded this new sketch to the arduino, yet sadly the same issue is happening of constant speed and direction in the wheels.
In terms of the PID libraries, I uploaded the PID_v1.h to the arduino library but the MPU6050_6Axis_MotionApps20.h would not download to the library, so I copy and pasted this in a new tab under the same sketch (which I believe is correct use of the angle bracket and quote notation for the #include function). Could this potentially be an issue?