I have Windows 7 and CS2. I just updated the plugin to Camera RAW 3.7, followed the directions to a T and still cannot open the NEF files. The D3100 is on the list of supported cameras.
The answer's in the fine print.
The D3100 is supported from Camera Raw version 6.3+
http://helpx.adobe.com/creative-suite/kb/camera-raw-plug-supported-cam eras.html
CS2 is only compatible with Camera Raw up to 3.7
http://helpx.adobe.com/x-productkb/global/camera-raw-compatible-applic ations.html
What else do I need to do to be able to work with the NEF files in CS2?
As a CS2 user, you must either use the DNG converter or upgrade to CS6.