Having Issues getting Featured image/thumbnail to show

I have read through the other thread regarding showing thumbnails, but it quickly turns into how to get excerpts to show properly without REALLY resolving the thumbnail issue.
My issue is, I have the plugin installed and working. You can see here that i have a link to a post showing on the page but I need the featured image for that post to display as well. In the other thread i see that it was suggested that this line of code… `$the_thumbnail = get_the_post_thumbnail($single->ID, ‘thumbnail’);
$output .= $the_thumbnail;`
be added, but it does not say to which file or where in the file. I also see that that thread is older, so perhaps it’s outdated.
If anyone can help me figure out how to get the featured image to show, that would be awesome.
Also, is there a settings page for the plugin, or is it simply controlled by the sections within the page or category admin areas?

Thanks,
-Jake

https://www.ads-software.com/extend/plugins/page2cat/