Hi evensenj
To show the iframe, you need to go to Backend > Systems> Plugins. Find the plugin called Editor-TinyMCE and click on it. When the settings page opens, find the text field that is called Prohibited Elements. You will see – script,applet,iframe – written inside the box. Delete iframe from this box so that the final text reads: script,applet.
I have helped you delete this, you can embed the iframe now. Also, I have added the demo iframe module, with a vimeo video, and tested it on your site.