Using Finder, locate a .kmz file on your system.
Ctrl-click on the file and select Get Info from the context menu.
Locate the section in the Info panel that appears titled: Open with:
Select the application you would like to handle files with that extension (.kmz in this case). If you don't see your application listed, there is an Other...
choice at the bottom that will allow you to find the application via browsing in a Finder-like interface.
Then click the button just below that titled: Change All...
and then click Continue
when prompted with "Are you sure... blah, blah, blah."
All your .kmz
files should now open with the application you chose. If that was Google Earth, you'll likely be happy at this point.