Your camera did not exist when CS2 ceased to be supported by Adobe, so you will never be able to open your Nikon D-300 files directly in CS2.
You need to download the DNG Converter 4.3 —previous versions do not support your new camera— and convert your files to DNG raw files first.
However, in CS2, you MUST run ACR (Adobe Camera Raw plug-in) 3.7, which does NOT support your camera directly but will convert the DNG raw files that you can get from your Nikon raw files through the separate, stand-alone DNG Converter 4.3 which doesn't any version of Photoshop to run. It runs as an application by itself. Place it anywhere you like, as on your desktop if you wish.
Again, download ACR 3.7 to install in CS2 according to the detailed instructions on the download page —but DISCARD the DNG Converter that may come with that download.
Then, download the free, stand-alone Adobe DNG CONVERTER 4.3 —but DISCARD the ACR 4.3 that may come with that download.
Alternatively, upgrade to CS3, upgrade its ACR to ACR 4.3 and open your files directly, without needing the DNG Converter at all.
Your other choice is to use Nikon software.