There is a thread
here for checking compatibility of your camera with the different versions of Adobe Camera Raw, and your CSx/Elements.
Unfortunately, it looks like the version of ACR which made the 5D Mark II compatible with ACR, v5.2, is not compatible with CS3.
I think you have a few options;
Update to CS4, then the latest version of ACR.
Buy the latest version of Elements which will enable you to use the latest version of ACR, once updated.
Find different RAW software.
Convert your RAW files to DNG files using the Adobe DNG converter which will allow your current version of ACr to read your files
Or of course, shoot Jpeg only.