Amazon Pay (Checkout v2) extension for OpenMage requires you to have a valid Amazon Payments account and a webserver running an OpenMage store instance with the following conditions met:
OpenMage 19.4.4 or higher
cURL for PHP
DOM / XML for PHP
valid SSL certificate
This extension works with OpenMage 19.4.4 or higher. Please refer to Installing the extension on Magento lower than 184.108.40.206 to find out how to install it on the legacy versions of Magento.