I've been trying to process raw files from my 5D MK2 using the latest version of dpp 3.5.1. I download images using a card reader, when I start dpp and navigate to the folder on my desktop and then select it I get the thumbnail view of the files in the folder but the images are not visible! I can see the box where the image should be and the "raw" icon and also the shot number are all there but the image is invisible. I can see the images when I use the canon file browser utility so I know there is an image with that shot number. When I select one of the "invisible" images as if I were going to process it a larger window opens with nothing visible and I get the spinning ball of death and have to force quit the app. I'm using a Mac G4, 2 gigs of ram on Tiger 4.11. I also tried to process raw files from my 40D and have the same problem. It seems this version of DPP is not happy on my Mac. I normally use Lightroom but it does not yet support the 5D MK2. Any suggestions would be appreciated.
Check the preferences in DPP; make sure it is set to your current monitor profile. I've had similar behavior in DPP if it points to the wrong profile or to one that no longer exists.
Barring that and repairing permissions, as mentioned above, you could also try trashing DPP's preference files (in your OS X User Library, e.g. com.canon.Digital Photo Professional.plist). You'll need to resent all of your DPP preferences after you do so.