Xbox 360 backwards compatible games are now able to be purchased via an Xbox One console in the game marketplace. All Games with Gold free monthly games are available to download directly from the Games with Gold advertisement on the main screen of the dashboard. Also, all backwards compatible games already purchased through the Xbox 360 can be found in the "Ready to Install" section at the far right side in the "Games" tab of "Games and Apps".
You can only purchase titles that are backwards compatible through the xbox.com website or via an Xbox 360 console. For example, if you are an Xbox Live Gold member, you can go to xbox.com/discounts and "purchase" the free monthly Xbox 360 games. As soon as you purchase those games on the website, you can go to your "Games and Apps" section on your Xbox One and download the new game through the "Ready to Install" section at the far right side of "Games and Apps".