Obviously this is a legal copy u have right! All dmg files should open using Disk Copy. If you double click it, it should open via disk copy. The other alternative is to drag & drop the dmg file onto disk copy.

Just a point. Normally you should be restarted via a startup disk before you install an os disk (such as disk 1.) So I hope this doesn't cause problems. Another alternative, is to burn the dmg to disk. Disk Copy can burn cd's. Open DC, go to DC's Prefs & click burning to set the speed, etc. Now drop the dmg onto DC, go to menu bar & select Image & go to Burn or press Cmd+B. Put a blank disk into your computer. Toast can also burn & mount many dmg files. So u might want to use Toast instead.
Hope these suggestions help.