How to enable/activate Airplay on iPhone 3G 4/iPad/iPod?

I don't yet fully understand how Airplay works. Do I have to enable or activate it somewhere. I'm trying to send my pictures and videos to be displayed on my apple tv player. But I just can't seem to make it work. Tried on both the iPhone and iPad.

tags: iPhone 3G, 4, iPod

You shouldnt have to.. All you have to do is make sure you have an Airplay enabled device (Airport Express or AppleTV), have the device connected to your wireless router, make sure your iphone/ipad/ipod is connected to the same wireless network and you should see a small arrow when playing your videos/audio/photos that will allow you to play it via AirPlay.. Hope this helps!