1. To get Demo XML
Download the sample data zip file, it is available on the account page, in the list of available downloads.
Unpacked can contain multiple files: all-in-one demo content or/and separately files for pages content, posts content, etc.
2. Activate Plugins
You should to remember before actually uploading the demo data is to install and activate your post type plugins (Portfolio, etc.) which are used in the theme.
3. WordPress Importer
Head to your WordPress Dashboard and locate the Tools sidebar menu item. Within this menu, you will see the Import link. Clicking this will bring up the Importer options – Click the WordPress Importer.
4. Install the Importer
If you have not activated the plugin WordPress importer you will need to install and activate it.
5. Upload the XML file
Now that your plugin is activated, you’re good to upload the XML file. Choose XML file and click Upload file and import to import the demo data, then click Submit.
See also WP Codex Page – Importing Content WordPress