1.sending part
#include <SPI.h>
#include <Mirf.h>
#include <nRF24L01.h>
#include <MirfHardwareSpiDriver.h>
#include <Keypad.h>
const byte ROWS = 4; //four rows
const byte COLS = 4; //four columns
//define the cymbols on the buttons of the keypads
char hexaKeys[ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};
byte rowPins[ROWS] = {7, 6, 5, 4}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {3, 2, 1, 0}; //connect to the column pinouts of the keypad
Keypad customKeypad = Keypad( makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS);
void setup(){
  Mirf.spi = &MirfHardwareSpi;
  Mirf.csnPin = 9;
  Mirf.init();
  Mirf.setRADDR((byte *)"clie1");
  Mirf.payload = sizeof(unsigned long);
  Mirf.config(); 
}
void loop(){
  char customKey = customKeypad.getKey();  
  if (customKey){
    byte time = customKey;
    Mirf.setTADDR((byte *)"serv1");
    Mirf.send((byte *)&time);  
    while(Mirf.isSending()){
    }
  }
}

2.receiving part
#include <SPI.h>
#include <Mirf.h>
#include <nRF24L01.h>
#include <MirfHardwareSpiDriver.h>
#include <Wire.h> 
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27,16,2);
void setup(){
  Serial.begin(9600);
  lcd.init();
 lcd.backlight(); 
  Mirf.spi = &MirfHardwareSpi;
  Mirf.init();
  Mirf.setRADDR((byte *)"serv1"); 
  Mirf.payload = sizeof(unsigned long); 
  Mirf.config();
  lcd.print("   ICSTATION");
}

void loop(){
  byte data[Mirf.payload];   
  if(!Mirf.isSending() && Mirf.dataReady()){
    Serial.println("Got packet");
    Mirf.getData(data);
    Serial.println(char (data[0]));
   lcd.setCursor(0, 1);
    lcd.print("The key is ");
    lcd.print(char(data[0]));
  }
}