The USB-HID Library is built into Arduino, there is no external installation required. Verify that you have a Metro Express, then copy and paste the code below into the Arduino Editor. Then, compile and upload.

/*
 * USB Blog Buddy
 * a USB-HID Scroll Wheel for Metro Express
 * 
 * by Brent Rubell for Adafruit Industries.   Support Open Source, buy Adafruit!
 */
 
// include the mouse library 
#include <Mouse.h>

// trimpot pin
const int trimPin = A0;

// button pin
const int buttonPin = 2;

// reduces scrolling speed (ms)
const int scrollDelay = 100;

// trimpot value
int trimValue = 0; 

// button state
int buttonState = 0;

void setup() {
  // start serial monitor at 9600 baud
  Serial.begin(9600);
  // start the mouse 
  Mouse.begin();
}

void loop() {
  // read the button state 
  buttonState = digitalRead(buttonPin);
    
  if (buttonState == HIGH) {
    // stop the mouse if button not pressed
    Mouse.end();
  }
  else {
    // start the mouse (if stopped)
    Mouse.begin();
    // read the trimpot value
    trimValue = analogRead(trimPin);
    // map the trimValues to scroll wheel down (-neg values) and up (+pos values)
    trimValue = map(trimValue, 0, 1023, -5, 5);
    // move the mouse wheel (dont change cursor position) 
    Mouse.move(0, 0, trimValue);
    // reduce the scrolling speed
    delay(scrollDelay); 
  }
}

/*
 * USB Blog Buddy
 * a USB-HID Scroll Wheel for Metro Express
 * 
 * by Brent Rubell for Adafruit Industries.   Support Open Source, buy Adafruit!
 */
 
// include the mouse library 
#include <Mouse.h>

// trimpot pin
const int trimPin = A0;

// button pin
const int buttonPin = 2;

// reduces scrolling speed (ms)
const int scrollDelay = 100;

// trimpot value
int trimValue = 0; 

// button state
int buttonState = 0;

void setup() {
  // start serial monitor at 9600 baud
  Serial.begin(9600);
  // start the mouse 
  Mouse.begin();
}

void loop() {
  // read the button state 
  buttonState = digitalRead(buttonPin);
    
  if (buttonState == HIGH) {
    // stop the mouse if button not pressed
    Mouse.end();
  }
  else {
    // start the mouse (if stopped)
    Mouse.begin();
    // read the trimpot value
    trimValue = analogRead(trimPin);
    // map the trimValues to scroll wheel down (-neg values) and up (+pos values)
    trimValue = map(trimValue, 0, 1023, -5, 5);
    // move the mouse wheel (dont change cursor position) 
    Mouse.move(0, 0, trimValue);
    // reduce the scrolling speed
    delay(scrollDelay); 
  }
}