You can either download one of the stable releases or the development version on the GitHub repository.
If you prefer to work with the code on the GitHub repository, simply enter the following to checkout the code:
A collection of plugins have been developed to extend OpenDial with new modules.
Starting from version 1.4 of OpenDial, all plugins are included by default in the release package, so you don't have to download anything. The external libraries for the Mary TTS, Sphinx ASR and MaltParser plugins are, however, only included in the "full package".
If you own a Nao robot and would like to implement some bindings between the NaoQi API and OpenDial, you can check this code.