The raw format for each camera model must be supported individually due to camera profile differences, and Adobe only updates their current versions of software when adding new camera support.
Basically, your camera is newer than your Photoshop. Specifically, the D7100 was first supported when PS-CS6 was the current version of Photoshop, and ACR 7.4 or ACR 8.x is required for support.
Besides upgrading to PS-CS6 or subscribing to the PS-CC+LR-CC deal for $10/month if done by Dec 31st, you can download and install and use the latest Adobe DNG Converter to create DNG raws from your NEF raws, which will open in the older CS5 ACR plug-in. Take care to set the ACR compatibility options low enough to work with ACR 6.x that CS5 uses. The default is ACR 7.1.
The DNG Converter is available, here – you need at least DNGC 7.4: