Apologies for replying late.
Select a dmg file, CMD+I, and see what it says for Open with. If it's not DiskImageMounter.app, then click on the drop-down menu, select Other, select your HD from the drop-down menu and navigate to System/Library/CoreServices and select it and click the Change All button. Now try to mount it.
If incase the above step doesn't work then the chances is the file is corrupt , may be becuase of failed download , hence re-download and check.
