Make sure thay are both on the same wifi network and Home Sharing is turned on on both devices. Your computer will need to be awake. If you do not have an Airport extreme, express or time capsule, you will need to manually wake the laptop.
Here's more on Home Sharing
waking your computer automatically.
Hi - having the same issue. Does it matter what Mac OSX one is using on the Macbook, in order to use Apple TV? I am on Mac OSX 10.8.5 on one laptop -- and the Airplay icon appears automatically in my menu bar. We have an older Mac that runs 10.7.5 and Airplay does not appear anywhere. I followed the instructions to turn Home Sharing on in iTunes.... but this doesnt allow the laptop to be projected onto Apple TV.
Any suggestions would be great!