Are you using the premium version of Softaculous ?
Joomla is not included in the Free version of Softaculous.
You can refer to the following guide on how to purchase Softaculous premium :
http://www.softaculous.com/docs/Buying_Information
If you have already purchased a license please make sure the scripts are enabled from Softaculous Admin panel -> Software
Here is the guide to enable/disable scripts :
http://www.softaculous.com/docs/Enable/Disable_Scripts