It is not a cgi folder but a ccgi server that is being referred to.
PlusNet have 2 webservers, one for running simple .html pages and one to run .php pages.www.username.plus.com
points to the www or homepage server which cannot run php code. FTP access is via ftp.plus.net.
ccgi.username.plus.com points to your ccgi webspace which can run .php as well as .html. FTP access is via ccgi.plus.net.
You have to enable the ccgi on your account via the website settings on the member centre page.
Have a look at the help & support pages related to ccgi and the usertools tutorials (see link in sig) which explain about the shell access (command prompt) to the ccgi webspace.