It is available in PRO version.
You should check in “Enable Similarity Marker Categories (pro)” checkbox in General tab of Settings / WP Related Items (WRI) menu.
Then you will find “WRI Similarity Maker Categories” menu items under all of the post type menus for which relationship handling is enabled.
For example, if you set that you would like to use WRI for posts and products (e.g. WooCommerce products), then you will find the “WRI Similarity Maker Categories” menu in “Posts” main menu and “Products” main menu. It is indifferent which menu do you use, because the content of them are the same.
I hope this information is helpful.