Ah--stuffit shouldn't be used to open .dmg files.
Drag the .dmg onto Disk Utility (in your system folder)
Better yet, Get Info on the .dmg file (command-I), and change the "Open with" option from stuffit to Disk Utility. If "Disk Image Mounter" is an option, choose that.
Make Disk Utility the default, and all should be well for the future too
