Here's how:

1. Log in to your PayPal Developer account.
2. Go to Testing Tools > Sandbox Accounts.
3. Find the email address of the business account you specified in your application.
4. Select the (…) button on the side > View/Edit Account.
5. Copy the email address.
6. Log in to the account on Sandbox using the email address. 

Note: The default password is "11111111." However, you can click Change Password in the developer portal to set a different password for this account.