Excerpt Box Not Showing in Post Editor

I would like to create a summary for some of my posts because even with using the More option, they are not always displaying well. Formatting of headers are lost and in the search results page, the spacing is also lost. I keep seeing reference to the excerpt for posts. For some reason, the excerpt box is not appearing on my post edit screen. How do I make it show up?

I’m using the Andrina Lite theme, WP version 3.5.1 and WordPress SEO by Yoast version 1.3.4.4. The site is https://comfortclimateservice.com

Thank you.