KMZ files are just gzipped versions of KML files and their constituent parts. Cartographica can read the constituent parts, but not the .KMZ file itself. So, to turn it into the constituent parts, do the following:
- Create a new folder in which to store the files and copy the original KMZ file there
- Rename the .kmz to .gz using the finder (you will be warned, but you really do want to change the extension)
- Double-click the .gz file and the Finder will decompress it
Now, you can open the kml file in its new location in the folder you created.