Many thanks for the prompt reply on a Sun night!!! I figured it out. It’s AUTOMATIC, if there’s more text. This is a great theme and simple theme for blogging. I’ll be sure to be asking more help if I can’t find it on the support pages.
I am having this exact same issue, but am not finding the solution like the original poster despite looking at the guide link you provided. The button will not appear despite having more than 10 words for excerpt.