Type this following command in your SSH Client(Putty or PenguiNet)
cd /usr/local/cpanel/whostmgr/docroot/cgi
wget -N http://www.softaculous.com/ins/addon_softaculous.php
chmod 755 addon_softaculous.php
Now go to:
WHM(Root) >> Plugins >> Softaculous - Instant Installs
If you can't do this then please create a Ticket
