Ohh ok i see :). The way you have it setup right now it is not really possible to select multiple categories i am afraid, as the category links in the left sidebar only link to taxonomy page.
To allow searching by multiple categories you would need to install a Search By Category snippet https://github.com/simpliko/wpadverts-snippets/blob/master/search-by-category/search-by-category.php it will add a multi-select dropdown to the search form. This will allow searching by multiple categories at once.
The snippet you can install by either pasting it in your theme functions.php file or by installing as a separate plugin (as explained here https://github.com/simpliko/wpadverts-snippets/ at the bottom of the page).
