just downloaded a rental to my laptop - how can I watch on Apple TV?

Hello astoklas,

You should be able to stream the rental to your Apple TV with the Airplay function. Here is some more info about this function and you can verify your computer meets the requirements for it:

iTunes: Using AirPlay

http://support.apple.com/kb/HT5922

To stream music or videos from your computer to an Apple TV or AirPort Express:

1. Connect your computer and Apple TV to the same Wi-Fi network.
2. Open iTunes.
3. Locate and click the icon to the left of the iTunes LCD window.
4. Select your Apple TV, AirPort Express, or AirPlay-enabled speakers or receivers from the list of available devices that appears and begin playback.

Note: The images of a TV screen or speaker next to each AirPlay device in the AirPlay menu indicate the type of media content you can stream to that device.

Thank you for using Apple Support Communities.

Regards,

Sterling