This isn't an ImageMagick problem. You would be better off asking this question in a group, such as the 10D group on yahoo - http://groups.yahoo.com/group/Canon-10D/.
However, I may be reading more into your post than is there, but I think you have to transfer the images from the camera onto your computer's hard drive first and then convert them. I have a 10D but I don't use any of the canon software to transfer/view/convert the raw images - I used Canon's software for about a week and gave up on it and got Downloader Pro to transfer from the CF card to the harddrive and BreezeBrowser for viewing/conversion - both from http://breezesys.com. If I need to process the images any further I use Paint Shop Pro - I can't justify the expense of Photoshop
Best Wishes
Pete