Currently Being ModeratedJan 13, 2012 11:55 PM (in response to Eastneuk)
I don't know why you can't open the .dmg, but is it your intent to install Norton anti-virus software on your mac? If so, don't. It's neither necessary nor useful.
Anti-virus software is unnecessary on the Mac except if you are running a Windows installation, in which case you might want something to scan your downloads. Don't waste money paying for Norton, which has been known to cause problems on Mac's. Use the free ClamXav if you're going to use anything. The full version is free, and its easy to remove/uninstall if you don't need it.
If you're going to use ClamXav, note that there are two versions: the App store version and the version on the developers website. Both are free, but only the second one contains the background scanner utility.