This plugin will display a countdown timer that counts down towards your next scheduled post.
For this plugin to create a countdown timer there needs to be atleast one post scheduled to be posted at a future time. If you do not have any posts scheduled then you will not see a countdown because there is nothing to countdown to. I would suggest scheduling a test post so you can play around with the plugin and get the formatting how you want it.
When you have completed writing a post, you have the option of posting it right away or scheduling it to post at a later time. You can do this by clicking on the "edit" link beside "Publish immediately" which is just above the save and publish buttons.