This is a quick example I whipped up to show how to integrate Cardinal Cruise Standard with PayPal.
You can read more here, but the long and short of it is this: you only need to integrate 3D Secure if both your acquirer and the issuer are in the European Economic Area (EEA). If you're a PayPal merchant, then this only applies to you if you're a UK merchant using Website Payments Pro. If you're a merchant in any other country, or if you're using a PayPal-hosted checkout product (such as Express Checkout or Hosted Sole Solution), then you don't need to integrate 3D Secure directly -- PayPal will take care of it for you.
You'll need to create a config.php file. Look at config.sample.php for instructions.
If you're using Website Payments Pro (which will be 99% of UK merchants), look at the example files in the paypal folder.
If you're using Payflow, look at the example files in the payflow folder.