This extension utilizes 2Checkouts back office API and Instant Notification Service to provide you with the ability to do all of your sales management from Magento. Additionally, this extension supports the 2Checkout Sandbox, so you can use this extension with your 2CO Sandbox account. It has been extensively tested, is used by many 2Checkout clients and is updated with each Magento community release.
Install the extension through magento connect in your account.
http://www.magentocommerce.com/magento-connect/2checkout-payment-extension.html
Once installed, it can be configured under Configuration->Payment Gateways. If you run into any problems, please make sure you are using the 2.0 magento connect extension and you have Stable set as your preferred state in the magento connect settings.