Hiya, you need to do this from the google play app. I've included a link below from google play you might find handy. If you are not using wifi, just remember data charges will apply. On prepay this is €1 per day for 50MB. We also have prepay data bundles.
€4.99 for 200MB for 30 days - text 5NET to 50231 or €9.99 for 700MB over 30 days - text 10NET to 50231