Now that I I remember clearly, the demo site which we used to import sample data is down for nearly a year. I mean demo.mangabooth.com. And mangabooth team (till version 1.7.3.1 - the version I use) didn't make any changes to system responsible to import demo site data.
Actually there's a way to import sample data but I'm not recommending it tho,
It'll CREATE AN OUTPUT BUFFER! so be careful!
However here's how to do it:
First you should know every function goes through pluggable and template they're connected. In short we're creating another fresh output buffer Wich ignores the original one. Here's how to do it- (DO IT ON A FRESH MADARA THEME NEVER EVER DO THIS ON A LIVE WEBSITE)
Cpanel/file-manager/WP_includes/pluggable.php
And add this code= ob_start(); after the first line. It should look like this,
<?php
ob_start();
(if you're using Madara child MK don't use this method it'll break lots functions)
Save and exit, go back to WP and install the sample data plugin and install demo from Madara dashboard.