The Geant4-DNA extension is fully included in the Geant4 public distribution.
Refer to the Geant4 installation instructions.
Alternatively, for users who do not wish to install Geant4 by themselves, we also propose a Geant4 Virtual Machine for VMWare™ or VirtualBox™ for Windows™ or Mac OS™, freely available for download at this link.
You may follow the updates of our Geant4 Virtual Machines on Twitter @ Geant4VM
Note : our source code is located in the $G4INSTALL/source/processes/electromagnetic/dna directory of Geant4.