Hi!
It depends on your theme structure and functionality. You will most likely have to add the search PHP shortcode into the results page file. This file is located in your theme folder, it’s usually the search.php file.
There is a short chapter in the documentation on how to use the PHP shortcode in themes: https://wpdreams.gitbooks.io/ajax-search-pro-documentation/content/getting_started/search_shortcode_in_themes.html
Unfortunately the plugin cannot place itself automatically there, as it is not possible in WordPress.