if it is not collected by paypal?
We're actually writing the the help article on this right now. In the meantime here's the short version. First, make sure the "$pp_checkscript" variable points to the the "pp-checker.php" file on your webserver. For example:
$pp_checkscript = 'http://example.com/buy/pp-checker.php';
Then, when your user has finished ordering through PayPal, PayPal then talks back to your "pp-checker.php" and gives it all the information (who ordered, the email, the amount, etc.) The "pp-checker.php" verifies the information, generates a product key using the LimeLM API, then sends the product key to the user.