Sample data for Magento 2.0 can be installed with 2 simple commands before Magento 2.0 installation:
composer config repositories.magento composer http://packages.magento.com
composer require magento/sample-data:*
How to install the sample data after Magento 2 is already installed: http://devdocs.magento.com/guides/v2.0/install-gde/install/sample-data-after-magento.html
You will get an authentication error, and it is normal: http://devdocs.magento.com/guides/v2.0/install-gde/install/sample-data-after-magento.html#authentication-error