Please put the code in the current active theme functions.php file: 

function my_wpdiscuz_shortcode() {
    $html = "";
    if (file_exists(ABSPATH . "wp-content/plugins/wpdiscuz/themes/default/comment-form.php")) {
        ob_start();
        include_once ABSPATH . "wp-content/plugins/wpdiscuz/themes/default/comment-form.php";
        $html = ob_get_clean();
    }
    return $html;
}
add_shortcode("wpdiscuz_comments", "my_wpdiscuz_shortcode");

Then put the shortcode in post content:

If you need to use shortcode in theme files add it with do_shortcode function:

<?php
   echo do_shortcode("");
?>

I'm using the free version for Brizy.

My plugins are all updated to the latest versions. 

1. I completed the setup for wpdiscuz
2. Then I added the shortcode to the page where I want the plugin to be. 
3. I also 'turned on" the comments for the page where I added the plugin,

Still, can't see the plugin, instead I see the text for the shortcode, see here 

I used another short-code for a different plugin, and it worked, but not for Wpdiscuz. 

Let me know what went wrong and how can this be fixed.