Paypal: "Add New 3rd Party Permissions" for Marketpress

Paypal is asking me to "Please enter valid API username" before it will let me request my Paypal API Credentials to enter into my store settings.

They say "Enter the API account username of the third-party that will be making API calls on your behalf. If you don't know the API account username, contact your shopping cart or solution provider."

What should I enter?