I originally posted this article to x.com on August 10, 2010. Since that time, x.com has been repurposed, and my posts have been taken down. I have reposted this here for informational and historical purposes.
Update 9/16/2014: I’ve tried to update this information to reflect the myriad of different account layouts that are available as of today.
One of the very first things that you must do when implementing Website Payments Pro, Express Checkout, Mobile Checkout, Adaptive Payments, or Adaptive Accounts is to get your API credentials. For someone like me, who has done this a hundred times, this is pretty simple; but I get calls almost every day from new merchants and developers who just don’t know what API credentials are, and where to go to get them. Further, in all my browsing through paypal.com and x.com, I don’t think I’ve ever seen a clear, concise guide on how to retrieve your API credentials. So, here’s the answer.