Before you can install Softaculous, you need to enable ioncube loader for PHP. This can be done in WHM (root) > Server Configuration > Tweak Settings > PHP. Select ioncube check box for cPanel PHP loader and click save button. This will enable Ioncube for you in 3rd party PHP binary.
To install softaculous, run
wget -N http://files.softaculous.com/install.sh chmod 755 install.sh ./install.sh
This will take some time as it need to download all software it auto installs.