Hi Eugenio,
1. First to generate EAN-8 Barcodes, In the plugin’s settings run the option under: Tools > Generate EAN for all products and select EAN-8
2. To see the barcodes in the admin page enable the option under: General > Admin products list column Once enabled you will be able to see the EAN numbers in the All products page in the WordPress dashboard.
3. To print the barcodes, in the plugin’s settings Enable the option under Print > Print barcodes (PDF) Now in the All Products page you will have an option under Bulk actions to print the barcodes in pdf.