If you get that message when opening the downloaded dmg file (disk image), then the downloaded file is corrupt: download again.
Wondering why you are running 10.6.3 instead of 10.6.8 - a typo, or have you not been installing all Apple Software Updates?
Cheers,
Karl
One of the forum moderators
AC 27 USA and earlier • macOS Ventura 13.6.6, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB