Display Category Titles

Hi,
1. the title is hidden by your theme CSS, you would need ask your theme support how to have the title displayed.

2. the slugs you can customize using this code snippet https://github.com/simpliko/wpadverts-snippets/blob/master/custom-slugs/custom-slugs.php

1. Ok.

2. Where should I paste the code snippet?

2 you can add it either in your theme functions.php file, or create a new WP plugin (as explained here https://github.com/simpliko/wpadverts-snippets at the bottom of the page) and paste it there.