The current versions of Adobe Lightroom and Camera Raw, and Apple Raw can’t open the raw/CR2 files from the Canon EOS-1D C. However, since these files are the same as the EOS-1D X, changing the camera model identifier in the file from C to X will allow them to be opened.
If you’re a Mac user, here is an Automator app (1DX2C.app) that you can just drop a CR2 file onto that will make the change. The app runs this perl command:
perl -i -pe 's/\x24\x03\x00\x80/\x69\x02\x00\x80/' filename
that substitutes the C’s model id 0x80000324 with the X’s 0x80000269
If you’re using Windows, I’m sure there’s something similar to run the perl command over the file.
When Adobe and Apple release a new version you can just change the files back to the correct id if you’re really keen.