You can use the Custom element to add an iFrame to your page.
The iFrame will not appear while editing and in preview; iFrame will load an http URL, but since iGloo works on a SSL (https) server for security reasons (this is the browser default behaviour), the http resource can't be loaded. So the issue is that you can't see the iframe on both edit and preview pages.
The iFrame will work when the page is exported to WordPress, and to your web-host or server.
You can also try and do these steps on your browser to view the iFrame while editing: