Hi!
I believe you will have to exclude all the other product categories, and leave only the “Mačka” and it’s subcategories.
More information on how to do that: http://wpdreams.gitbooks.io/ajax-search-pro-documentation/content/advanced/excluding_categories_&_terms.html
Also, make sure that the “product” custom post type is selected for searching: http://wpdreams.gitbooks.io/ajax-search-pro-documentation/content/general_settings/search_in_custom_post_types.html