XBMC changed it name to Kodi with the 14.0 update. Also the website now is: http://www.kodi.tv
To update from XBMC to Kodi 14.0 follow these steps:
Login to your apple tv:
ssh root@your.apple.tv.ip
password: alpine
Once loged in as root type following commands to install it:
apt-get update
apt-get install org.xbmc.kodi-atv2