How to mount a DMG image created with dd? I created an "emergency" image of my dying HD using the procedure explained in 
Recover a dead hard drive using dd.

Now when I try to mount the dmg file I get the message: "The following disk image couldn't be opened... no mountable files"

How can I mount the image and explore the directories and files?
I've gotten shafted doing a similar rescue using dd. I doubt this will help, but it's worth a shot: How to Repair DMG Files. Another comment on there reported success opening a problematic dd created image using filesavage, seems kinda dated and pricey for a maybe.

I don't know if your drive is still alive or not, if you can do another rescue run with different software, that would probably be your best bet. 

In the comments on the webpage you linked to, there were suggestions of using ddrescue, a worthwhile read here before you go down that road and it might explain why your (and my) dd backup did not work. 

You can easily install the current version of ddrescue if you have macports installed sudo port install ddrescue. Failing that, If you have xcode installed, it's trivial to download the latest source code, browse to download in terminal, extract lzip -dc /PathTo/ddrescue.tar.lz | tar -x, cd to extracted ddrescue directory, then make and sudo make install. If not, another alternative is the this ddrescue-gui that I came across, it has ddrescue bundled within the application, you'll want to download the .dmg version. This might be an easy way to go, but you lose the power of log files (you can merge multiple partial recovered images).

I've used testdisk/photorec in the past with some success, it got some of my files but not all of them, and it was not organized at all by the way, many duplicates. I'd try ddrescue or ddrescue-gui out if I were you, if you want to do the free &amp; command line route. As always, read the manual, it's got some good practices as far as best recovery practices. Dated Mac OSX hints entry

Seek and ye shall find. Best of luck, let me know if you need help with anything.