Adding IFRAME to a Plone Document
You have to allow iframes in the ZMI and your HTML-filter.
1. Go to ZMI-->portal_transforms/safe_html and add "iframe" at the bottom of
the list with a "1" value.
2. Go to your HTML filter settings in Plone and add "iframe" to your custom
defined / allowed HTML-tags.
3. Restart Plone.