Next to API Access, click Update.
Scroll down to NVP/SOAP API integration (classic) and click Manage API credentials.
Click the radio button beside Request API signature then click Agree and Submit.
Click Show next to the API Username, API Password, and Signature. Copy them to your clipboard.
Click Sources on the left side navigation pane then click +New Data Source. Find PayPal in the gallery and enter the API Username, API Password, and Signature you retrieved in the last step.