A growing number of users are experiencing issues concerning Mac OS X 10.6 Snow Leopard's handling of .dmg files. Since upgrading to Snow Leopard these users are not able to mount the files or run packages stored in them.
An Apple Support Discussions thread reports on the issue. User "Logan G" writes:
Since upgrading to Snow Leopard, I cannot get any downloaded .dmg files to do anything. After downloading them, I double click on them and nothing happens. Nothing. No error message. No bouncing dock icons. Nothing. I've tried downloading them using Firefox and Safari with the same results. This is happening with all .dmg files, not just the same ones, so I know it's not unique to a particular .dmg file.
Several suggestions are made regarding a fix for this issue, although none seem to permanently solve the problem. First, check to be sure your Disk Image files are set to open with the correct program--in this case, DiskImageMounter. To check, highlight a disk image file and press (Command + I) to Get Info. In the Open with: section, if DiskImageMounter is not selected, choose it and click "Change All..."
You can also try deleting the following files:
Home > Library > Preferences > com.apple.frameworks.diskimages.uiagent
Home > Library > Preferences > com.apple.diskimages.fsck
You will then want to log out, and in again or restart your Mac. This solution seems to solve the issue temporarily, though users report that the problem returns. You may also be able to open the disk image files using Disk Utility (Applications > Utilities). Note: Always take caution when deleting preference files. Be sure to have a stable backup of all you important files.