Moving this discussion to the Camera RAW forum.
This discussion probably belongs in the Lightroom forum, not the Adobe Camera Raw forum which is mostly about the ACR plug-in for Photoshop, but I’ll answer, anyway:
LR has support for many Nikon cameras. How are you judging the lack of support? What are you doing that doesn’t work how you expect it to?
As a first guess, if you have a newer Nikon camera and are using a very old Nikon Transfer 1 or View NX, something that came with an old Nikon camera to copy your NEF files from your camera to your hard-drive then the obsolete Nikon software is corrupting your NEFs so that LR doesn’t recognize them.
The solution for ne4w NEFs is to either use a version of View NX2 that is new like your camera, or stop using Nikon software to transfer your files and use Lightroom, or just use Finder of Explorer to copy the NEFs to where they should be on your hard-drive and import from there.
Another guess is that you’re trying to open an NEF with LR5 instead of using LR5 to create a LR catalog database, and then using LR’s File / Import to instruct LR where the NEF files are.
This person did ask this question in the Lightroom forum, and was asked for some clarification. No further response came.
Sorry, it was night here
Well at first I expected that LR was able to read NEF files without a RAW converter. This was correct. Somewhat undiscovered problem within the Windows 7 is causing the problem, it is reporting the file to be corruted while it is a administrator issue.
So for now nothing to blame LR or Adobe.
I think case closed, and thanks for the comments which kept me thinking about other things.