Billy I've solved my problem and probably yours.
Log in on dropbox and click here dropbox.com/enable_public_folder
The new accs doesn't come with public folders and as new users we are inclined to think that a shared folder is a public what is not the case.
After you have enabled the Public folder you gonna be able to export your project to it and see it working.
Ps.: If you are reading this and have the power, please fix the tutorial section about dropbox preview.