To install ckanext-citeproc:
Activate your CKAN virtual environment, for example:
. /usr/lib/ckan/default/bin/activate
Clone the source and install it on the virtualenv:
git clone https://github.com/open-data/ckanext-citeproc.git
cd ckanext-citeproc
pip install -e .
pip install -r requirements.txt
Add citeproc to the ckan.plugins setting in your CKAN config file (by default the config file is located at /etc/ckan/default/ckan.ini).
Restart CKAN. For example if you've deployed CKAN with Apache on Ubuntu:
sudo service apache2 reload