So you have a few Apple computers at home each having its own iTunes Library, or perhaps you have just one Mac but it has more than one iTunes Libraries. How do you access their contents from your AppleTV?
Firstly, you must already have set up a “Main” library. If not, go to AppleTV’s “Settings -> Computers -> Connect to iTunes [if it shows your Mac’s name then your AppleTV is already connected, for which you’ll skip the next step.]. Follow the on-screen prompt where it instructs you to key in the Passcode in your Mac’s iTunes. Done. Your AppleTV will Sync its contents (ie, download the selected music, videos, photos etc) onto its internal hard disk. Having contents Sync-ed means you can play your music and videos etc directly on your AppleTV without turning on your Mac, or have iTunes running in it.
To access your other Libraries, be it on another Mac on the same network (or your alternative Library on the same Mac,) you must first setup the Sharing option in your other Mac’s iTunes’ Preferences -> Sharing -> (tick) Share my library on my local network. You can choose to share the entire library or selected playlists, and even enforce Password required access.
Now go to your AppleTV -> Settings -> Computers -> Add Shared iTunes Library, and follow the on-screen instructions by entering the Passcode in your other Mac‘s iTunes (or the alternative Library of your lone Mac.) Finally, select the newly listed Mac from the Computers screen and you’re done.
To enjoy the shared media (they are not synced/downloaded but streaming live instead) such as Music, go to AppleTV -> Music -> [and scroll down to] Shared Music, and start enjoy your streaming music! Do likewise for your Movies, Photos, TV Shows etc, but select the “Shared whatever” option on the bottom. Your streams will continue so long as your other Mac is running. If the “Shared …” option is not available, go turn on the Mac and/or launch its iTunes.
If AppleTV does not seem to be able to stream from your other Mac, your Mac’s (or router’s) Firewall may be blocking access. Just temporarily turn off Firewall protection and try again to see if it’s the culprit.
Enjoy your AppleTV.
 To achieve that, simply hold the Option key before clicking on the iTunes icon in your Dock to launch iTunes. A pop-up Menu will appear asking you to choose the Library, or create a new one. The purpose of having different libraries is primarily to separate contents, such as keeping your adults’ only material beyond preying eyes.