How do I get my PayPal API username and password?
For PayPal Website Payments Pro and PayPal Express Checkout you need to obtain your API credentials. Here is how you do that:
1. Log in to your PayPal account
2. Click the Profile subtab
3. Click on 'My Selling Tools' in the left column
4. Click on API Access under the selling online tab
5. Choose Option 2: "Request API Credentials"
6. Then request API Certificate
More info can be found here or via PayPal Support.