You can install Plainbox straight from the archive:
$ sudo apt-get install plainbox
Plainbox can be installed from a PPA on Ubuntu Precise (12.04) or newer.
$ sudo add-apt-repository ppa:checkbox-dev/ppa && sudo apt-get update && sudo apt-get install plainbox
Plainbox can be installed from pypi. Keep in mind that you will need python3 version of pip and you will need to have python3-lxml installed (package names may differ depending on your platform):
$ pip3 install plainbox
We recommend using virtualenv or installing with the --user option.