adding plug in to widget menu

Hello, I have installed your excellent plug in and activated it, but even though it is showing up as installed on my primary widget sidebar in the admin screen, it does not appear on my actual site. I am running the Twenty Fourteen theme, and am probably missing something really daft- but I can’t find what as the other widgets and plug ins that I have dragged into there are all showing. Thanks

https://wordpress.org/plugins/this-day-in-history/