Dear Eli,
You have check the box on screen option of any post or page to let the excerpt box appear on screen.
Dashboard->All Pages->Pages->Choose any page
Click on Screen Options-> Check the box on Excerpt-> Go downwards there you will find the box for inserting text on excerpt.
See the image below:
http://prntscr.com/losp5p
http://prntscr.com/lospqv
Best Regards!