In order to add an iFrame to your page, use the custom element feature inside the builder.
The iFrame will not appear while editing or in preview. The iFrame will load in an http URL, but since iGloo works on a SSL (https) server for security reasons (this is the browser default behaviour), the http resource won't load.
So, after adding the iFrame, while you won't be able to see it in the preview or in the builder, to preview it in action, please export the Page to WordPress, or your web-host or server.
You can also try take these steps in your browser to view the iFrame while editing: