Im not sure if I tagged it right but I think its ok. Anyway we are having trouble with this and was wondering if there would be a certain code to do this. Thanks for any help at all. 
Here is some boilerplate code to get you started.
// Left Motors and motor group
motor leftMotorA = motor(PORT1, ratio18_1, false);
motor leftMotorB = motor(PORT2, ratio18_1, false);
motor leftMotorC = motor(PORT3, ratio18_1, false);
motor_group LeftDriveGroup = motor_group(leftMotorA, leftMotorB, leftMotorC);
// Right Motors and motor group
motor rightMotorA = motor(PORT11, ratio18_1, true);
motor rightMotorB = motor(PORT12, ratio18_1, true);
motor rightMotorC = motor(PORT13, ratio18_1, true);
motor_group RightDriveGroup = motor_group(rightMotorA, rightMotorB, rightMotorC);
// Drivetrain control
distanceUnits units = distanceUnits::in; //Imperial measurements - inches.
double wheelTravel = 4 * M_PI; //Circumference of the drive wheels (4" x PI)
double trackWidth = 18; //Distance between the left and right center of wheel.
double wheelBase = 15; //Distince between the center of the front and back axle.
double gearRatio = 1; //Ratio of motor rotations to wheel rotations if using gears.
//Drivetrain code if using an Inertial Sensor
inertial DrivetrainInertial = inertial(PORT21);
smartdrive robotDrive = smartdrive(LeftDriveGroup, RightDriveGroup, DrivetrainInertial, wheelTravel, gearRatio);
//Use the following Drivetrain code if NOT using an Inertial Sensor
//drivetrain robotDrive(LeftDriveGroup, RightDriveGroup, wheelTravel, trackWidth, wheelBase, distanceUnits::in );
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