Nikon D3100 needs Adobe Camera raw version 6.3, in order to work with PS CS 5
Please check the ACR version on your PS CS 5
1. Open PS CS 5
2. If its Windows OS, then click help >> about plugin >> camera raw
if its mac, then click Photshop menu >> About Plugin >> Camera raw
3. Check the version of the camera raw, if its less than 6.3, Nikon D3100 will not work
In order to update your Camera Raw, please update your PS CS 5 from the Help menu
This is the document where all the cameras along with their supported ACR versions are mentioned : http://helpx.adobe.com/creative-suite/kb/camera-raw-plug-supported-cameras.html
Once, ACR is updated to 6.3 or greater (6.7 is the maximum version) then , the pics from Nikon D3100 will work and open without any issues.
Moving this discussion to the Adobe Camera RAW forum.