First add the repository for XBMC:

Stable XBMC:

sudo add-apt-repository ppa:team-xbmc/ppa

Beta and release candidate versions:

sudo add-apt-repository ppa:team-xbmc/unstable

Nightly builds:

sudo add-apt-repository ppa:team-xbmc/xbmc-nightly

Now install dependencies and XBMC:

sudo apt-get update
sudo apt-get install python-software-properties pkg-config software-properties-common xbmc