RAW files are opened (in Photoshop, Elements) by a sub program called Adobe Camera Raw. For every new camera model that comes out, ARC has to add the code for that model, otherwise it can't open the raw files. That's why ACR is updated two or three times a year.
So it's very likely that your ACR just needs to up updated. Go to Adobe.com and find the latest version of ACR for Elements 9.
If you have a very new camera model, it may not be supported yet. In which case, you'll have to use the software that came with your camera to open and convert the RAW file. You can save it as JPEG, TIFF etc., then open it in Elements.