First, you need to install the App. You do it the same way as with any other Joomla extension.
Once the App is installed, go to
Extensions->Plugins-> System - DJ-Classifieds Ajax
and open the plugin.
Next:
Assuming you have enabled all options, the App changes how those things work:
Ajax is used for any action that is performed in DJ-Classifieds Search module. That works with text search and anything you can click on the module.
Ajax calls are used to change the pagination.
Ajax calls are used to sort and order items.
Display slim loading bar at the top of the page, during Ajax calls.
Show the reset/link button for clearing the search form fields.
Ajax is used to open the categories.