Example shown below is with verion ISPConfig 2.2.15.
1) go to the /tmp dir
2) download the file (with wget)
3) unpack the file
tar xvfz ISPConfig-2.2.15.tar.gz
4) step into the directory that the ISPConfig-2.2.15.tar.gz is unpacked in
5) run setup
ISPconfig will create all the needed directories.
Make sure that you are root when doing all this.
Login with your username, and type
It will now ask for the root password.
More info here: http://www.ispconfig.org/manual_installation.htm
(scroll down about 1/2 a page, and you will see the install info)