Connect header (5×2) to connect to LCD & Keypad extension board.
Use 4 digital line for data transfer to LCD, 3 digital line to control LCD, and 1 analogue line to transfer information of key pressed.

Digital Output Pin: 23, 25, 27, 29, 31, 33, 35 (BackLite, EN, RS, DB4, DB5, DB6, DB7)
Analogue Input Pin: A3 (Keypads)
Power Pin: GND, +5V (see supported function)

LCD_Keypad_DL_V1_0_0.ino

/* LCD & Keypad example
 
   Board: DATA LOGER ARDUINO MEGA SHIELD, 24.11.2011, V 1.0.0
   Author: Dubravko Penezić 2011, Creative Commons BY-NC-SA
 
   Used pins:
     DOP-23, DOP-25, DOP-27, DOP-29, DOP-31, DOP-33, DOP-35, A3
 
*/
 
// LCD pins
int BeLi_LCD_pin = 23;
int RS_LCD_pin = 27;
int En_LCD_pin = 25;
int D4_LCD_pin = 29;
int D5_LCD_pin = 31;
int D6_LCD_pin = 33;
int D7_LCD_pin = 35;
 
// include the library code:
#include <LiquidCrystal.h>
// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(RS_LCD_pin, En_LCD_pin, D4_LCD_pin, D5_LCD_pin, D6_LCD_pin, D7_LCD_pin);
 
//Key message
char msgs[6][15] = {"Right Key OK ", 
                    "Up Key OK    ", 
                    "Down Key OK  ", 
                    "Left Key OK  ", 
                    "Select Key OK",
                    "             " };
//Analogue value for key
int  adc_key_val[6] ={30, 170, 390, 570, 790, 1024 };
//Number of keys
int NUM_KEYS = 6;
//Analog pin
int aKeyPin = 3;
 
//other variable
int adc_key_in;
int key=-1;
int oldkey=-1;
 
void setup() { 
  pinMode(13, OUTPUT);  //we'll use the debug LED to output a heartbeat
  // set up the LCD's number of columns and rows: 
  pinMode(BeLi_LCD_pin, OUTPUT);
  digitalWrite(BeLi_LCD_pin, LOW);
  lcd.begin(20, 2);
  // Print a message to the LCD.
  lcd.print("KEYPAD testing press");   
}
 
void loop() {
 
  adc_key_in = analogRead(aKeyPin);    // read the value from the sensor  
 
  digitalWrite(13, HIGH);  
 
  key = get_key(adc_key_in);	               // convert into key press
 
  if (key != oldkey) {	               // if keypress is detected
    delay(50);		               // wait for debounce time
    adc_key_in = analogRead(aKeyPin);  // read the value from the sensor  
    key = get_key(adc_key_in);	       // convert into key press
    if (key != oldkey) {
      if(key < 5) digitalWrite(BeLi_LCD_pin, HIGH);
      else digitalWrite(BeLi_LCD_pin, LOW);
 
      oldkey = key;
      if (key >=0){
        lcd.setCursor(0, 1);
        lcd.print("                    ");
        lcd.setCursor(0, 1);
        lcd.print(msgs[key]);
        lcd.print(" ");
        lcd.print(adc_key_in);
      }
    }
  }
 
  //delay(1000);
  digitalWrite(13, LOW);
}
 
// Convert ADC value to key number
int get_key(unsigned int input) {
 
  int k;
 
  for (k = 0; k < NUM_KEYS; k++) {
    if (input < adc_key_val[k]) {
      return k;
    }
  }
 
  if (k >= NUM_KEYS)
    k = -1;     // No valid key pressed
 
  return k;
}