In order to use PayPal Express Checkout, you need to have a PayPal business account, a standard PayPal account will not work.
Once you have a business account follow these steps:
1. Log into your PayPal account
2. Click "My Profile" under the "My Account" tab
3. Find "My selling tools"
4. Find and click on "API access" (see image 1)
5. Click on "Enable Express Checkout" (see image 2)
6. Make sure you select "Accept PayPal payments before API permissions are set up" (see image 3)
7. Click on the Submit button to save the setting
8. You should now return to your PayPal profile with a message saying that your preferences have been saved.