Your Photoshop CS5 is too old for that camera. You’d need at least CS6 updated to ACR 8.7 to open D750 NEFs natively.
You can subscribe to the cloud license of Photoshop, where the cheapest is the Photography Plan that also includes Lightroom, at $10/month (US), and get something that’ll work as long as your operating system is new enough, or you can use the free DNG Converter 8.7 or newer to create DNGs from the NEFS, one folder at a time, and the DNGs, but not the NEFs will open.
Free DNG Converter is available, here: http://www.adobe.com/downloads/updates.html
The Photography Plan is here: https://www.adobe.com/creativecloud/photography.html