You can install Mambo on your account via the automatic script installation system - Softaculous. Here is the way of installing Mambo from Softaculous:

cPanel --> Softaculous --> Portals/CMS --> Mambo --> Install

You can also request the script to be installed on your account by SiteGround Technical Support Team. They would gladly assist you with the installation of any script/application that is compatible with your hosting account.

Log in to your Customer Area --> Get Support Section --> HelpDesk --> Get script installed -> Script Installation