After you download a disk image ( .dmg ) file, the file should either mount automatically or mount when you double-click on it. If it doesn’t, the Finder might be attempting—and failing—to open the .dmg file using a seemingly random application. (When this happened to me, the Finder had decided to try Adobe GoLive CS.)
To fix this problem, select the image file, hold down the option key, and select File -> Always Open With. In the menu that appears, select DiskImageMounter. If this application is not in the menu, select Other and navigate to the /System/Library/CoreServices folder. You’ll find the DiskImageMounter application there.