Mounting a .dd Image on OS X
There's a built in command called 'hdiutil' that allows you to mount image files. You can read more about the command in its entirety by entering 'man hdiutil' in Terminal.
The command needed to mount an image file is:
hdiutil attach [options] /path/to/image.dd
There are several options you can include (they're all listed in the man page for hdiutil), but one of the most common is '-readonly'. This is a really important option to include if mounting a .dd image that is for a forensics investigation to make sure the image isn't modified at all.
If you have any other questions, let us know!