To activate it, you need to drag widget from Available widgets area to Sidebar you want it to be displayed. Just go to Appearance >> Widgets and have a look at a screenshot bellow:
To add searchable fields, please click on Add search field - Select field and on the field you want to add.
You can drag and drop the fields to change their order on frontend:
Show/hide settings will allow to enable widget on specific pages:
You can choose which page will be used for search results. By default it's automatically created WP Search Page.
But if this page needs to be customized, you can create a new page with the shortcode [es_search] and choose it as a Search page - and you will be able to add widgets, listings or any additional information you want.