The plugin enables any user to display summarized information on any of the posted items.
When you write or edit the content of a page, simply include [blog_activity] (along with the brackets) whenever you want the table to be displayed. Make sure you activate the plugin before you use the shortcode.
The plugin probably has not yet been activated.
At the moment, the blog-activity-shortcode plugin only works when used in pages. The content displayed by the plugin table probably malfunctioned if you used the shortcode in a post.