I have tried all of the CHDK file extension options CRW, CR2, and DNG, and none of them are visible on this Win 7 box.
To use "DNG visible via USB" you must use the DNG extension. All it does is make ".DNG" one of the cameras recognized extensions, replacing one of the little used ones. You must also use a prefix that doesn't confuse the camera firmware. CRW_ should be OK. IMG_ is likely not OK.
You may also be able to download DNGs if you use a prefix / extension already recognized by the camera (e.g. CRW_nnnn.CR2 or CRW_nnnn.CRW) without enabling "DNG visible via USB"
Of course I checked that "DNG Visible via USB" is enabled, and "Remote" is disabled, and I restarted the camera before plugging in the USB cable. Is "DNG visible via USB" a function that is not supported at all in 64 bit Windows?
CHDK doesn't know or care about the host OS. On my A540, "DNG visible via USB" works fine under windows 7 64 bit, using CRW_nnnn.DNG for the filename.
An alternative is to download your files using the CHDK extension http://chdk.wikia.com/wiki/PTP_Extension