You have a few options to play Pandora on Apple TV.
First Option :
Jailbreak your apple tv or jailbreak your apple tv 2. Unfortunately there is no option for jailbreaking an apple tv 3 yet. After you jailbreak your apple tv, you need to install XBMC. XBMC is a great software for playing all kinds of content without limitations. Install the the pandora repository or the pandora add-on for XBMC. Now you can play XBMC directly from your apple TV.
Second Option (requires an ios device):
AIRPLAY! if you have an ios device, get the pandora app for your device and simply airplay it to your apple tv. This is by far the easiest method if you infact have an IOS device.
Third Option :
Ditch the apple tv and upgrade to the Taffy Media Center. Then you can enjoy all the above options pre installed and ready to go with a faster interface and higher quality video!