Have you activated the plugin and no posts are showing up when viewing the widget, or perhaps no posts are being returned by the custom function?
99% of the time this is caused by a lack of data. WP Most Popular needs to gather data before it can rank and return posts. The quickest way around this is to click through to a post on your blog. This post will now be ranked as a view has been recorded. It will now also show up in the list of popular posts via the widget or custom function.
If you are still not seeing any data after visiting a post on your blog, something has gone wrong. We recommend deleting the plugin and reinstalling it. If still nothing, then please log a bug with our support team.