Before you get started with PayPal integration, note that to do auto-recurring subscriptions with PayPal you must have a PayPal Website Payments Pro account; a basic PayPal account does not support the necessary functionality for Spreedly to run unattended charges.
PayPal has several different kinds of API access, but only one of them will work with Spreedly. This guide will walk you through finding and entering the right set of credentials.
Notes:
First, go to your PayPal profile and click on the API link:

Next, go into the PayPal API settings:

We need a certificate, so drill into that:

We need a certificate:

The three key pieces of information you need are the API Username, the API Password, and the Certificate file:

Once you have all three in hand you're set to plug them into Spreedly and start collecting money.
Drill down into the payment gateways configuration:

Add a new gateway:

Select PayPal:

Now fill in the three fields with the three pieces of information you gathered from PayPal earlier:
As a final step, select which payment methods to use with PayPal:
You should now be all set to go. As mentioned above, we highly recommend you run at least one real money transaction before exposing Spreedly to your customers.