Currently there is no Zattoo App available on the Apple TV. You can, however, use stream Zattoo from your iOS or OS X device using AirPlay.
Please make sure that your device is logged on to the same wireless network as the Apple TV box. Otherwise, the Airplay indicator will not be displayed.
AirPlay functionality is not available for ProSiebenSat1 channels in Germany. This is due to technical reasons which are connected with the lately implemented DRM protection for those channels.
For helpful tips for AirPlay synchronization, see the following pages:
iOS: Troubleshooting AirPlay and AirPlay Mirroring.
iOS and OS X: Recommended settings for Wi-Fi routers and access points.