You will need to configure the PHP script to send the appropriate values to PayPal for processing. Even though it does currently redirect to PayPal, you need to do some additional configuration to ensure all the values passed are correct.
Open up the /includes/library.php file and edit the following lines, found at the very top.
For the success_url, cancel_url and notify_url values, you need to set them to the location of those files - included in the zipfile - relative to your webroot. The default examples assume you've uploaded the zipfile into a "donate" folder.