That’s easy, you need ACR 7.4 or newer which only works with PS-CS6 or newer:
The most direct option is to upgrade to CS6 or get a CC subscription; otherwise, you can use the DNG Converter 7.4 or newer (8.7.1 is the current version) to create DNGs from your NEFs and those should work ok in the older CS5/ACR6.
Thank you!!! I would love to be able to update to Creative Cloud but it's not quite in the budget haha. Sorry for a dumb question, but what is a DNG? Is it already on my computer, or do I need to download it? Thanks again!
DNG = Digital NeGative. New camera support requires new profiles. In this situation a DNG is Adobe’s combination of the original raw data and newer camera profiles, so that older ACR plug-ins can then use to decode the data.
The DNG Converter is available at the Downloads / Updates link I gave, earlier. Download and install it, then run it on a folder of NEFs and convert that folder to DNGs. Those DNGs will then be visible in the older Bridge/ACR.
You can get PS-CC for $10/month with the Photography Plan and it would be a pretty tight budget that can’t afford that, but ok: