Download a release of the software. The homepage is http://DirectDemocracyP2P.net (update packages are at: http://debatedecide.org/DD).
Store and unzip/untar the system in a folder. Moving the files later will require re-entering the paths in the control panel. Java 1.7 is required, and users of Linux amd Mac need to install in advance
sqlite3 (they are installed automatically by the Windows installer). Ad-hoc scripts for Linux also need g++.
Start the program using the dd_p2p.bat (Windows) or dd_p2p.sh (Linux/Mac) scripts in the obtained folder.
In case you want to recompile it from sources, sources are available at github.
A plugin with a Spacecraft Game is available on https://github.com/Ryan-Knowles/P2P_Space_Game.