Move your mouse over the Tools menu in top navigation. Find More Tools within that. And click it.
Scroll the page till you see a box with API credentialswritten. Click on it.
Once you've created the API credentials, you will see View API Signature option under NVP / SOAP API integration. Clicking on that will bring you to a page like the following screenshot. Click on each "Show" link to see API username, password and signature.
Still can't find the API details?
If the above instructions did not work for you, try this:
Login to your PayPal account
Click on Profile
See the classic Profile version (on the left side, outside the My Selling tools box)
API Access (under Account Information)
Choose Option 2 – Request API credentials – click on View API signature.