In order to use S-Drive, you need to create your own S-Drive Portal account. Once you install S-Drive in your Salesforce org, you will login to this portal acount and connect your org. Your portal can connect multiple orgs--sandboxes and production. In the portal, you will also be able to see your Amazon account, your S-Drive invoices, and payment details. Go https://portal.sdriveapp.com. Register for a free account, or login if you already have an account.
For more information about your Portal account, including how to register a payment method, see payment details, download invoices, connect or reconnect to organizations, change your password or Account information and more, see S-Drive Portal Account