How do I enable my Sandbox test account for PayPal Payments Pro?

Here's how: 

1. Log in to your PayPal Developer account. 
2. Click Sandbox accounts near the bottom of the page. 
3. Click the email address for the account for which you would like to enable Payments Pro. 
4. Under Sandbox account info, click Upgrade to Pro next to Account type. 
5. Click Change to Payments Pro. 

PayPal Payments Pro can only be anabled for US, UK and Canadian business accounts.