So, it's been a little while now since I set this up, but here's what I did (which worked):
You need to open the display-posts-shortcode.php file in the plugin folder. Find the // Set up initial query for post // section. This is where you'll be adding to the existing code and inserting the query that allows for pagination (this is a copy of what I created before the "// If Post IDs" section).
[Code moderated as per the Forum Rules. Please use the pastebin]
Next - you need to add pagenavi. I inserted it after the 'endwhile;' like so:
wp_pagenavi( array( 'query' => $listing ) );
I'm no expert at this stuff, but I was able to hack a bit to get this working. I hope this helps you.