Trouble with serial port: HAL - VISA: Resource Not Found

[techplex]

Greetings, we hare having trouble using our NavX MXP via its USB connection.

We have our NavX connected using a USB cable as described:
https://code.google.com/p/navxmxp/wi..._via_USB_cable

When running the sample code https://code.google.com/p/navxmxp/so...eRobotExample/

We get the following exception:

ERROR Unhandled exception: java.lang.NullPointerException at [
org.usfirst.frc.team2465.robot.Robot.operatorControl(Robot.java:93), 
edu.wpi.first.wpilibj.SampleRobot.startCompetition(SampleRobot.java:148), 
edu.wpi.first.wpilibj.RobotBase.main(RobotBase.java:234)]

The error appears to be referring to the first use of the imu object.

boolean is_calibrating = imu.isCalibrating();

Further investigation reveals that the SerialPort may be the culprit.
We moved the following line of code into the robotInit method yields a more descriptive error:

(Robot.java:81)

SerialPort my_serial_port = new SerialPort(57600,SerialPort.Port.kUSB);

ERROR Unhandled exception: java.lang.RuntimeException:  Code: -1073807343. HAL - VISA: Resource Not Found at [
edu.wpi.first.wpilibj.hal.HALUtil.checkStatus(HALUtil.java:48), 
edu.wpi.first.wpilibj.SerialPort.<init>(SerialPort.java:198), 
edu.wpi.first.wpilibj.SerialPort.<init>(SerialPort.java:251), 
org.usfirst.frc.team2465.robot.Robot.robotInit(Robot.java:81), 
edu.wpi.first.wpilibj.SampleRobot.startCompetition(SampleRobot.java:121), 
edu.wpi.first.wpilibj.RobotBase.main(RobotBase.java:234)]

Any Ideas?