http://sitemile.com/
This tutorial was made using ProjectTheme: http://sitemile.com/products/wordpres...
You can follow this tutorial for every other SiteMile Themes, the setup process is the same.
Step by Step Tutorial:
Step 1 - Go to developer.paypal.com and sign in to your regular PayPal Account
Step 2 - Go to Dashboard / Sandbox / Accounts
Step 3 - Create a Seller Account ( I recommend to create a Business Account )
Step 4 - Copy the Credentials from your -facilitator Account to the theme's fields.
Make sure to add every detail from there, not from your regular PayPal, because sandbox will work only with the credentials copied from the sandbox accounts.
This tutorial is about how to setup sandbox for testing the transactions on your site.