In general, you can call SmartDashboard.putNumber in an init method (so that it gets loaded on the SmartDashboard), and then call SmartDashboard.getNumber to read the current value on the Smart Dashboard.
If you are using the PIDController class, there is already a PID widget on Smart Dashobard that lets you change P, I, and D, and the setpoint. You can add the PIDController as an actuator with LiveWindow, and test whatever you want.
See:
http://wpilib.screenstepslive.com/s/...ewindow-values
http://wpilib.screenstepslive.com/s/...smartdashboard