I will gladly submit a support ticket for you if need be. I think you are very close to getting the correct credentials from PayPal, you just need to proceed with obtaining the API signature and other values. I'm including some screen shots of the screens that I go through to get this setup.
You should not need to worry about granting any third party access as you are setting up the checkout for your own account. You can skip over any of that and just request the credentials directly. Here are the screens that should get you the correct credentials.
Please post back and let me know if you have any problems getting the values.