can't open mpkg files on my mac

[johnscott333]

Hi,
I have a MacBook Pro 10.4.11, and lately my computer has not been able to install several programs I've tried downloading from online. The .dmg file will save fine to my Documents folder, but when I then open up the dmg, there is an installation .mpkg file.
When I click on the .mpkg, nothing opens or runs. What does happen is that, in the dock, the icon for Installer.app (which I have set at the default) appears for about a second, then disappears. This means I can't open the installation file, so the specific software can't run.
Id've done some reading online, and I think there might be something wrong with my Installer app. itself, but am not sure.
If this was the case, would I then need to get the original OS install CD's to reinstall Installer??
the problem is, I don't have access at the moment to my original installation discs....is there anywhere online to download a new version of Installer??
thanx
john