There are some issues with the Credentials not appearing for some accounts. Currently you are logged into the developer account. Try logging into the actual sandbox seller account, and going to the profile and requesting the credentials. You should be able to view these from within the Sandbox seller account, and if they have not been requested yet, you can generate these.
Log out Paypal account
clearing browser cache and cookies (I use firefox).
Go to developer.paypal.com and login with the real paypal account (not the Sandbox one)
Go to Applications
Click on Sandbox accounts
(Optional) Import your old accounts from your former Sandbox account after the redirect to the accounts page
Click on the account you want to log in to
Choose Sandbox site
Log in to that site using your test business account
Go to My Profile
In Account information click on Request API Credentials
Choose the option that solves your needs
The API crecendiales were created !!! (they will appear in your test account profile)