Make sure the proprietary Nvidia drivers are installed. You can find them in the "Additional Drivers" tab of the "Software Sources" app. If you don't see any Nvidia drivers listed, make sure you have the "Proprietary drivers for devices (restricted)" box checked in the "Ubuntu Software" tab and your system is up to date (search "update" in the Unity dash and run the software updater).

Once you've installed the Nvidia drivers, you no longer use the built-in display manager. Instead, you use Nvidia's. Just search for "Nvidia" and you'll find it. I don't remember the section name off hand, but it says something along the lines of "X Server Configuration," and when you go to it, it will look similar to the built-in display manager (showing two screens and allowing you to adjust their position, resolution, and whatnot). Make the changes and click "Apply" to test them out. Once you get the settings the way you want, click the "Save X Configuration" button to make the changes permanent. It will ask you for your password, since it's a system configuration change.