Jim
I had a similarish prob:
Try this:
Open existing Osirix, click Osirix---quit osirix
Remove Osirix icon from the DOCK BAR
Then FINDER---CMD+backspace on existing Osirix Icon (puts it into trash)----empty trash
Then double click the new version DMG file, will get a prompt to drag osirix icon to application
Drag the icon into applications
Launch Finder and navigate to Osirix--go one step further and you will see further subfolder plus another Osirix.app icon
When you double click this it will install the new version
If this fails probably your Mac is not compatible !
Let me warn you, with the latest version all your images will have--Not for Medical Use---- box as default !!!
Then you can download the old version (i would recommend 3.8.1) as you dont get the annoying Memory message with this version for larger CT datasets
You can chose not to get an update message by unchecking the boxes under Osirix-Preferences-General
Let me know how you got on
Cheers
Sameer
after downloading the O.5 version, got a prompt to move the osirix icon into the application folder.