How do I set up PayPal parallel payments in Drupal 7 Commerce Express Checkout module?
The short answer is that there isn't just a place to "paste the snippet." That integration wizard is hopelessly simplistic. Support for a new payment method within Drupal Commerce would require more than simply adding code to a page, because presumably you'll want some sort of local record of any payment action taking place. For that to happen, you have to work with the Drupal / Drupal Commerce APIs as well.

In Drupal Commerce, these integrations are handled through payment method modules. We have documentation for writing them in the developer guide on DrupalCommerce.org. Additionally, if you're PayPal Express Checkout, you should switch to using the 2.x branch of the main PayPal project, http://drupal.org/project/commerce_paypal, as it has a complete implementation of the Express Checkout payment method and has been verified by PayPal.