Photoshop uses a sub program to open/edit RAW files...it's called Adobe Camera RAW. Each camera's RAW files are different and you have to have a compatible version of A.C.R. for the specific camera that you use.
Updated versions of A.C.R. are avaliable on the Adobe web site, for free.
I'm not certain, but you may run into a problem in that the 400D is only compatible with versions of A.C.R. that are avaliable for Photoshop CS2 or CS3 (not CS)...in which case, you will not be able to open your RAW files with Photoshop unless you update to CS2 or CS3.
Your camera will have come with software for opening your RAW files. Just open them, make your basic adjustments then save the files in a standard graphics format like JPEG or TIFF. Then open them in Photoshop.
There are even better options for working with RAW files. Adobe Lightroom or Apple's Aperture, for example.