Let's say you have the leftDriveStick and the rightDriveStick. The right stick X and Y will be the forward/backward and strafe. The left stick X will be the rotate.
Code:
Joystick leftStick;
Joystick rightStick;
RobotDrive robotDrive;
float x = DEAD_BAND(rightStick.GetX());
float y = DEAD_BAND(rightStick.GetY());
float rot = DEAD_BAND(leftStick.GetX());
robotDrive.MecanumDrive_Cartesian(x, y, rot);