You can obtain the required API credentials by following the instructions provided below:
1) Log in to your PayPal Business account.
2) Click the My Account tab.
3) Click the Profile tab. If you haven't already done so, you need to verify your account before requesting API credentials.
4) Click "My Selling Tools", and then click the "Update" button located next to "API access".
5) Click "View API Signature".
6) Copy your API username, password, and signature and add them on the Merchant Setup section of the Payment Settings page of your form.