Make sure your mobile device is connected to the same Wi-Fi network as your Chromecast device.
To access Chromecast from the Pandora app, go to the Now Playing screen. Then tap on the Chromecast icon to the right of the song title, and your Pandora station should load on your TV screen. You will still control Pandora from your device.
Locate and tap this icon again if you want to disconnect Pandora from Chromecast.