Source code can be found here (see the note on the project home page):
https://usfirst.collab.net/sf/projects/wpilib/
One, often used, way of implementing a rotation PID using a gyro is to put it between the joystick and the rotation input of the RobotDrive mecanum method. Use the joystick to command changes in the angular setpoint and use the output of the PID as the rotation input of the mecanum drive method.