Andrea,
If you are attempting to embed an iFrame that uses the HTTP protocol on a page this is using the HTTPS protocol the iFrame will not display, as the browser will not load it.
You will need to request an SSL certificate for your CName so that you can use the HTTPS protocol when sourcing in iFrames. Please contact the services department (services@marketo.com) to setup the SSL certificate. You will be required to purchase your own SSL certificate and provide it to Marketo.