First thing to do is download the image again. And if you still get that error, or get that error with all images, than it’s time to try a few things.
Most of the time the error has to do with what’s called Launch Services, the way the OS X keeps track of what to do with files and programs. Sometimes the Launch Services database gets corrupted or is missing parts, especially after system software upgrades. And so when you double click on the dmg image and you get the error “The disk image wouldn’t mount,” it really means that OS X doesn’t know to use the built in program DiskImageMounter to mount the image.
So if you delete the old Launch Services preference files and caches and then restart, OS X will rebuild the Launch Services database and those disk images should work OK. What you do is:
1 – Restart and then don’t open any programs.
2 – Trash the following two files in the Macintosh HD > Library > Caches folder:
com.apple.LaunchServices-0140.csstore
com.apple.LaunchServices-014nnn.csstore“nnn” in the second file name is going to be your User ID number, but that probably doesn’t matter.
3 – Type your Admin password if prompted to delete those files.
4 – Restart your Mac and then empty the Trash.
Get a fresh download of the dmg you are trying to open and try it.
If that doesn’t work, trash the com.apple.LaunchServices.plist file in the Home > Library > Preferences folder of your account name and restart and empty the trash.
If you want to make it easy to rebuild Launch Services, you can get a utility called MainMenu to use for regular and preventative maintenance on your Mac. When you start it up, it will show up as an icon in the menu bar to the left of the Airport icon. Then you can pull down on the menu and select what you want to do, i.e., repair disk permissions, run the middle-of-the-night periodic tasks anytime, clear font caches if you have text display problems, and much more, including rebuilding Launch Services. It’s free for the first 15 days of use.
If all that doesn’t work, you’ll need to try and reinstall the updater for your version of OS X. Got to Apple and find what’s called the Combo Updaters. That type of update replaces more files than the incremental updates, lets you “leap frog” some middle updates, and many times will solve problems.
For examples, the Combo Updaters for 10.4.11 are this one for PPC and this one for Intel
If you’re using 10.5, search for the latest combo updater.