command not found means that the command you are running in your terminal (the first word entered before a space delimiter) was not found in your PATH variable. Your PATH keeps track of all the locations and directories where when you call 'virtualenvwrapper' it looks under all of those for a virtualenvwrapper file that it can run. Normally when you install with PIP, you already have the path where pip installs scripts to included in your PATH variable so you don't run into issues.
So the docs are saying that you don't run 'virtualenvwrapper mkvirtualenv <environ_name>' they say you should run 'mkvirtualenv <environ_name>'. This should work if you have already sourced the `virtualenvwrapper.sh` script and you have set up variables for WORK_ON and such as described in the docs.