Looking at the code, it appears that this will only work for Dropbox accounts that have a public folder (users who signed up prior to October 2012 or paid users

Looking at the code, it appears that this will only work for Dropbox accounts that have a public folder (users who signed up prior to October 2012 or paid users who have specifically enabled the feature). (See https://www.dropbox.com/help/16. https://www.dropbox.com/help/16.)