Iframe Preserver plugin allows you to easily embed any iframe and preserves your embedded iframes by utilizing WordPress' Custom Fields.
You can install Iframe Preserver directly from the WordPress admin panel. Visit Plugins > Add New and search for 'Iframe Preserver'. Click to automatically install. Once installed, activate and Iframe Preserver is functional.
The old school install for Iframe Preserver plugin:
Download and unzip the iframe-preserver.zip file
Upload the iframe-preserver folder to your plugins directory
Activate the plugin through the 'Plugins' menu in WordPress.
You are now ready to use the Plugin according to the directions in the description.