Website Payments Pro 3.0 uses the PayPal DoDirectPayment API and requires you to enter an API username, password, and signature. There are separate fields to enter both sandbox and live credentials, and then there is also a checkbox to enable Sandbox / Test Mode. If Sandbox mode is enabled then you do not need an SSL or Force Secure checkout enabled in WooCommerce to complete test transactions.
Payments Pro 2.0 uses the PayFlow API. This is what you would use if you were given a PayPal Manager account at manager.paypal.com. There, you login with a username, password, vendor, and partner value. You would use the same values in the plugin settings that you use to login to your PayPal Manager. If you don’t use a username value to login then you would need to use the same value in the plugin for both username and vendor.
Neither of those options, though, will add a PayPal Button because those are direct credit card options. Express Checkout is what adds the PayPal button, and that’s what allows people to sign in and pay through PayPal. Express Checkout would use the same API credentials that DoDirectPayment uses.
It’s always recommended to enable Express Checkout, but then you need to figure out which version of Pro PayPal got you on and only enable that one for Pro. In most cases DoDirectPayment will work fine for you.