The meta keys are _wti_like_count, _wti_unlike_count, _wti_total_count. By default all the existing posts wont have these meta data. When someone clicks like or unlike button for a post, then these meta data gets added/updated for that post. On adimn plugin settings page, you will see a button Update Like Meta. If you click this button, then it will update the above meta data for all posts in the system.