I had this problem and it took me hours to sort out!
It's dead easy.....ensure your apple tv is switched and showing on your tv
Make sure your iPad is connected to your wi-fi router
Make sure your apple tv is connected to your router
Now the bit that took me ages to find! Double click your home button on your iPad and apps appear at the bottom as usual! Swipe those apps to the right and a new sub menu appears with the apple tv icon showing.