Apple selbst bietet diese Loesungen an:
anstatt mit Doppleklick das dmg zu oeffen, erst Disk Copy oeffnen und das dmg mit drag und drop in das Fenster ziehen. Wenn das nicht hilft - wie von Andi schon beschrieben - die Permissions reparieren.
Die original url zu dem knowledge base article ging ueber 4 Zeilen, die id ist 107288
DISCUSSION
Symptom
A disk image file will not open. This may occur immediately after you download it.
Note: Disk image files usually have a filename extension of ".img".
Products affected
Mac OS 8
Mac OS 9
Mac OS X
Solution
Mac OS X
1. Open Disk Copy (/Applications/Utilities/).
2. Drag the disk image file icon into the Disk Copy window.
If the disk image file still does not open, users of Mac OS X 10.2 or later (Note 1) may follow these additional steps:
1. Open Disk Utility (/Appliations/Utilities/).
2. Select the Mac OS X startup disk in the left-hand column.
3. Click the First Aid tab.
4. Click Repair Disk Permissions.
After repairing permissions, try opening the disk image file again.
Notes:
1. If you have Mac OS X 10.1.5, you may use the Repair Privileges Utility, which is available from Apple Software Downloads (
http://www.apple.com/swupdates/). If you have Mac OS X 10.1 through 10.1.4, you must update to version 10.1.5 to use the utility.
2. If you use Aladdin StuffIt Expander, note that Aladdin recommends using Disk Copy to open disk image files in Mac OS X (
http://www.aladdinsys.com/support/techsupport/mac/) .
3. If an alert appears after you download a disk image file, you may need to configure your web broswer to open disk image files with Disk Copy. See technical document 106171, "Mac OS X: Double-Clicking a File Opens the Wrong Application".