XBMC for Apple TV is not just a remote or a client streamer, it is a full XBMC install. No longer will your Apple TV be just chained to iTunes. You will also get gull hardware decode for 720p and 1080p movies.
This method will only work on second generation Apple TV, and requires a jailbreak, which could be easily done using Seas0nPass.
Step 1: You will need to log in as root. To do this, open Terminal, and enter
ssh root@apple-tv.local
If apple-tv.local doesn’t work for you, enter your Apple TV’s local IP address.
Step 2: If asked for password, enter alpine.
Step 3: Now, enter the following code in Mac OS X’s Terminal. Enter one command at a time.
echo “deb http://mirrors.xbmc.org/apt/atv2 ./” > /etc/apt/sources.list.d/xbmc.list
apt-get update
apt-get install org.xbmc.xbmc-atv2
Step 4: Sit back and enjoy as XBMC is being installed on your Apple TV.
Step 5: After XBMC is successfully installed, Apple TV will restart automatically. There you will see XBMC installed.

If you don’t already have XBMC running on your computer download the newest build by selecting your operating system.