Did you activate the plugin? Also check plugin settings. It seems like you either didn’t activate plugin itself or something is not right in its settings. Some plugins will let you choose between comments on posts and pages, and some only work for posts.
See if yours has ability to display comments on pages and if it doesn’t try another plugin — they are so many!
Another thing to check if you have comments enabled in your WordPress setup. It is enabled by default, but maybe you have disabled it while working.
Finally some WordPress themes do not allow comments on pages so you also need to make sure your theme does.
I was able to get it to work by going to Pages, selecting all the pages, selecting edit from the Bulk Actions drop down menu, clicking apply button, choosing allow from the comments drop down, then clicking the update button. I then repeated for other pages.