To add an iframe to a page you will need to remove the first < and replace it with a [. Then at the end of it you will need to remove the ></iframe> and replace it with ]. Here is an example of what an iFrame would look like. You can then go in and change the width and height to any dimensions you would like.
Paste the shortcode in the Text section.
[iframe src=”https://www.google.com/calendar/embed?src=en.usa%23holiday%40group.v.calendar.google.com&ctz=America/Los_Angeles” style=”border: 0″ width=”800″ height=”600″ frameborder=”0″ scrolling=”no”]