Use the Servo class.
It has two methods you can use:
set(double value) where value = 0.0 to 1.0 for full range of motion
setAngle(double degrees) where degrees = position of the servo...values in excess of allowable range of motion simply saturate to the servo limit. The API assumes that the kit servos are linear.
So to answer your question. Since the joys run from -1 to 1, you can simply use a multiple for the set method:
Code:
Servo servo = new Servo(1);//Servo in PWM port 1
double servoIn = (joy.getX() +1)*0.5;
// Command to servo
servo.set(servoIn);
That should do it.