If you are switching from PayPal Mass Payments, then you should remove your IPN notification in PayPal. You can find the instructions to do that listed here.
Head over to PayPal's Developer portal and click on Dashboard in the dropdown located in the top right corner of the screen. You may be asked to login first.
You need to verify that you have the Payouts API enabled. Unfortunately, this might take anywhere from a few hours to several days for PayPal to give you permission. We don't have control over this. However, applying for it is very easy. At the bottom of the column called "permission name", you'll see the row labeled "Payouts". You want to click the "Enable" link.
PayPal will email you that they are reviewing your request. Once you've been accepted you'll notice a green checkmark signifying that the API is live.
Click on My App & Credentials in the left side menu. Scroll down till you find the "REST API apps" section found under the My Apps & Credentials. Click the "Create App" button.
(Create a name for your App) Under App Name type "Trolley" Payout. Then click the "Create App" button.
Click Show underneath the Secret header and make sure to select "Live" on the top right.