When you are logged in as root, you can give the command:
wget <URL at your nearest download location>
To find out your nearest download location, go to the download section at http://www.ispconfig.org
and press "Download" behind the text "Download ISPConfig 2.2.6 from sourceforge.net".
Then, press the link "download" behind your nearest location and you will find out the right URL for your download for using the wget command.
When you install ISPConfig, choose expert mode.
If you do so, you can choose where you want to store a www directory for storing the websites and so on. For example /var/www.
If you install it in the standard mode, it will use the default settings of your Linux distro.
All the instructions you will need can be found here: