Make sure to enter your credentials in the cloudinary.yml (github.com/taragano/Carrierwave_widget/blob/master/config/cloudinary-sample.yml). It can be downloaded from here: cloudinary.com/console/cloudinary.yml.
Also, set the cloud_name and the corresponding upload preset here: github.com/taragano/Carrierwave_widget/blob/master/app/views/posts/_form.html.erb#L35.