I've been unable to open some some software updates(non-apple) that have an .mpkg extension.
I've open the package contents, but that as far as I'm able to go. Any suggestions. I'm currently using OSX 10.5.8.

Normally .mpkg files are auto opening/running.

Found the following with a Google search

What happens when you double-click the mpkg file? Does some other application open, instead? If that's the case you need to re-associate .mpkg files with "Installer":

1. Select an mpkg file
2. Do a Get Info
3. Select open with, and choose Installer from the list, if it's available. Otherwise, open the Other option and find Installer (it should be in /Applications/Utilities).
4. Click Change All, and confirm when prompted.

Also check out this Google search for some possible help click here.