You cannot download the RAW images over a USB cable - your camera's software does not see them and so will not sent them to your computer.
This is not completely true. Whether you can see them depends on the prefix and extension you chose for the file, and also the particular camera and possibly the software on your PC. It's quite complicated so I can't give specific instructions for a specific camera. Generally, CRW_NNNN.CRW or CRW_NNNN.CR2 are the best bet. IMG_NNNN.* will generally NOT work. Files with the .DNG extension will not be visible, unless CHDK supports the DNG visible over USB option. You can save DNG format files without a DNG extension, although you may want to rename them once they are on the PC.
You must also restart your camera between shooting raw and plugging into USB, otherwise the files will not be visible.
You can also use CHDKs PTP extension (with ptpcam or ptpcamgui) to transfer files that are not visible in normal PTP. You can find these in the german forum
http://forum.chdk-treff.de/viewforum.php?f=7 Information about the CHDK PTP extension can be found in the PTP thread
http://chdk.setepontos.com/index.php?topic=4338.465
