You will get the "Unknown installation error " alert if you try to launch Reason from the mounted disc image. After downloading and mounting the Reason disk image (dmg) file, drag the entire Reason folder (not just the Reason app) from the disk image to the Applications folder.
Once you've copied it, eject the Reason disk image volume, then launch Reason from the Applications folder instead. It could also be a problem with your disk permissions. Open "Disk Utility" in the Mac OS Application/Utilities folder. Click "Repair Disk Permissions" and let it do its work. Also, make sure you are installing with "Admin" rights. You could also check the Read/Write permissions for the Reason app to make sure you have read and write permissions.