I've got some good news and some bad news.
The bad news is that the D810 didn't get out-of-the-box support until Camera Raw 8.6...which isn't available in your version of Photoshop. To get support, you'll have to upgrade to a newer version of Photoshop.
Otherwise, the good news is that you can download the free DNG Converter, which will allow you to convert those RAW files for use in Adobe apps.
However, you should know that the version of ACR that runs in CS5 will give you inferior image quality compared to the rendering provided by any version of ACR higher than 7.3, for which you definitely would have to upgrade.
It makes no sense to use an expensive camera like yours and stick with CS5. If you can afford that camera, you can easily afford to upgrade.