This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
Using this extension is quite simple. You start editing or creating a content element and select “External Media” as content type. Then you only need to provide the URL to the media you want to embed (e.g. a YouTube video) and save the content element.
If a provider is known for this URL the embed code will be retrieved and the media will be embedded. Otherwise an error will be rendered.
You can use the width and height parameters so specify maximum width and height of the embedded element.