After following the instructions on Doug Hellman\'s virtualenvwrapper post, I still could not fire up a test environment.
[mpenning@tsunami ~]$ mkvirtualenv
Solved my issue in Ubuntu 14.04 OS with python 2.7.6, by adding below two lines into ~/.bash_profile (or ~/.bashrc in unix) files.
source "/usr/local/bin/virtualenvwrapper.sh"
export WORKON_HOME="/opt/virtual_env/"
And then executing both these lines onto the terminal.