Deactivate and re-activate the plugin and then try adding the posts. There is a chance that the database has not been created for the plugin yet. Let us know in case of any issue.
Check if you are adding the post to Smooth Slider correctly on edit-post window. The correct way of adding a post to slider is to check the box which says "add this post/page to". And then select the slider (slider name background should change to blue color) in the box next to that check box. And then save/update the post.
