IDX searching requires a dsIDXpress plugin which is paid: http://www.diversesolutions.com/product/wordpress-idx/
IDX plugin is optional and it's only an addition fo people that use MLS systems that are supported by this plugin. Adding and managing listings is built in in the theme and you don't need to have any paid plugins for that. Everything can be made with WordPress panel: adding/deleting offers, managing search form, appearance, adding users/agencies/agents etc. Please take a look at the documentation to see how it works:
This plugin IS NOT REQUIRED to use the theme. It's only an addition for MLS users.