Where can I download the software from?

To download the software, you need to access their SVN repository. Issue the following commands to see which versions are available.
svn list https://svn-ccsm-release.cgd.ucar.edu/model_versions/
svn list https://svn-ccsm-models.cgd.ucar.edu/cesm1/tags

And then export the version that you need.