Are you saying that you're using this on a secure page (HTTPS) and you're getting this error?
It's very important to know that Facebook has discontinued the Facebook Registration plugin and it will altogether stop working as of April 30, 2015.
The Registration Plugin is deprecated and will be removed on April 30,
2015. You should not integrate this plugin into new apps. If you're using this plugin, you should replace it with Facebook Login before
April 30, 2015
Solspace Facebook connect still currently uses the old v1.0 Facebook API and sites that do not update before April 30, 2015 will lose all Facebook functionality. That said, we are finishing up a new version of Facebook Connect that will be released any day now and will be fully compatible with current version of Facebook (v2.2 API). However, as mentioned earlier, the Registration plugin will not work, due to Facebook discontinuing it. :)