Ok, first the bad news.

Adobe first provided support for the D300s raw files with version 5.5 of Adobe Camera Raw (ACR) which is a plug-in to all Adobe applications and which in fact is the part of the application that 'reads' raw files. However, (now the bad news) version 5.5 of ACR only works with Elements version 7 - not version 6 as you have. So - reading your camera produced raw files from within Elements 6 is a non starter.
The good news however

is that Adobe realised that various camera makers themselves were no longer providing support for their own, but older, camera produced raw files. Unbelievable, but true non the less. At the same time, Adobe (who have been fighting a long long battle with camera manufacturers in trying to convince them to adopt a universal raw format) released their own DNG format and converter. In short, you can convert your cameras native raw files (NEF?) into Adobe's DNG without any loss in quality. By the way, the DNG format is just another form of raw format so you are simply converting from one raw format to another. In fact, many photographers as a matter of course convert their camera produced raw files into the DNG format right from the beginning of their work flow.
So you really have 2 options - either upgrade to the newest version of Elements which as well as adding a few newer features will support your D300s or, if you are happy to remain with Elements 6 download the free DNG converter and give it a go.
If you go for the latter option, my advise to you would be to proceed as follows:-
1. Download and install the free DNG converter from here
http://www.adobe.com/support/downloa...jsp?ftpID=4679
2. Set up the Preferences in the converter as you see fit - they are few and well explained. One option is to embed the original raw file within the converted file. This may appeal to you at the beginning whilst you experiment to see how things go, but of course makes the newly created DNG file that much bigger. I personally don't embed the original because in all the time I have been converting my files I have yet to notice any drop in quality.
3. Create a Test file on your PC's desktop to use for experimental purposes and copy one of your raw files into this temporary folder. If all goes wrong then you won't have damaged anything in the process.
4. Open up the DNG converter and open up this copy of a raw file from the temporary desktop folder and proceed to convert it to the DNG format.
5. Open up the newly created DNG file with elements and your existing Camera Raw plug-in (whatever version) will recognise it. Then proceed with editing in the normal manner.
Best of luck - let us know what you decide and how you get on.