The D7100 is supported only in PS-CS6/ACR 7.4 and newer. There will be no update for CS5.1.
You can upgrade your CS to CS6 or CC or download the free DNG Converter to make DNGs from your NEFs and open those in older versions of Camera Raw. You’ll want to make sure to set the DNGC compatibility options to something lower than the default of ACR 7.1. The DNG Converter is available, here: