Unfortunately there is no package available for Orxonox yet. We're eager to release one, though. Until then, you can get a copy of Orxonox' source code and build it on your own. If you experience problems, see this page or contact us.
To install all tools and libraries Orxonox needs, type the following command (as root)
# aptitude install build-essential cmake libogre-dev libenet-dev \ libvorbis-dev libalut-dev libcegui-mk2-dev tcl-dev libboost-date-time-dev \ libboost-filesystem-dev libboost-thread-dev liblua5.1-0-dev libtolua-dev libtolua++5.1-dev
Now proceed by following the steps described on the building page.