These instructions are valid as of 5 May 2010 and may vary over time.  To create a test paypal pro account do the following:

1. Log in to your developer account at https://developer.paypal.com
2. Create a preconfigured normal account (not a paypal pro one)
3. Make sure it is a Seller account.  Copy the password for use later
4. Log in to the account at https://www.sandbox.paypal.com
5. Click on 'Products and Services'
6. Click on 'Website Payments Pro'
7. Click on 'Accept Billing Agreement'
8. Click on 'Agree'
9. Now select the option by which you want to connect to the API

The image on the left can be downloaded from the attachment