When you click on button and nothing happen, you miss javascript library.
Some themes does not use fancybox library, therefore you should add it into your theme's head.

Go to file:
oc-content/themes/{your theme}/head.php

After <?php tag place code:
osc_register_script('fancybox', 'https://cdnjs.cloudflare.com/ajax/libs/fancybox/2.1.7/js/jquery.fancybox.min.js', array('jquery'));
osc_enqueue_script('fancybox');
osc_enqueue_style('fancybox', 'https://cdnjs.cloudflare.com/ajax/libs/fancybox/2.1.7/css/jquery.fancybox.min.css');

Even if you experience problem after that, it's probably issue with compatibility of your jQuery library and fancybox library.

You still can test different libraries on site:
https://cdnjs.com/libraries/fancybox/2.1.7

There you replace link to .js and .css file with updated (or just change number) and try.