To setup paypal express checkout:
Login into your paypal business account.
Click the following link for a shortcut to your API credentials:
Then login to your website's admin area and go to:
1. Modules > payment
2. Remove the paypal standard payment module.
3. Install the paypal express payment module.
4. Enter your paypal API credentials.
To find your API credentials in your paypal account (instead of using the shortcut above), login to your PayPal account and go to:
1. Tools > Business setup.
2. Offer PayPal checkout on your website > setup online payments.
3. Choose option B.
4. Setup API access when you're ready to go live.
5. Find 'NVP/SOAP API integration (Classic)' > Manage API credentials
6. Click 'show' and copy the API credentials