It's difficult to be too much more specific.
My personal preference for zip programs is 7Zip (which is freeware). WinZip and WinRAR should be able to handle these files but 7Zip definitely can. The KMZ file uses a simple DEFLATE method for compression.
When you unzip the file, it should unzip to 2 files:
<RootFolder>
-"doc.kml"
-"files"
--<image file name>
The doc.kml, if you open it up in a text editor like notepad will show an XML structure like this one...a <GroundOverlay> containing an <Icon> reference (the image with a local - not absolute - reference) and a <LatLonBox> reference (the extents)
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="
www.opengis.net/kml/2.2" xmlns:gx="
www.google.com/kml/ext/2.2" xmlns:kml="
www.opengis.net/kml/2.2" xmlns:atom="
www.w3.org/2005/Atom">
<GroundOverlay>
<name>Untitled Image Overlay</name>
<color>96ffffff</color>
<Icon>
<href>files/chart-nz-5_1-0005-0003.jpg</href>
<viewBoundScale>0.75</viewBoundScale>
</Icon>
<LatLonBox>
<north>-36.38468155987549</north>
<south>-36.62718367028314</south>
<east>175.7278146808112</east>
<west>175.4837582938124</west>
</LatLonBox>
</GroundOverlay>
</kml>
In my case, I ran through a file "Waikawau.kmz" some kiwis sent me.
It broken down to:
"D:\"
-"Waikawau"
--"doc.kml"
--"files"
---"chart-nz-5_1-0005-0003.jpg"
after (optionally) running the keyhole mode, it expanded to:
"D:\"
-"Waikawau"
--"doc.kml"
--"files"
---"chart-nz-5_1-0005-0003.jpg"
---"chart-nz-5_1-0005-0003.jgw"
I can then add the "D:\Waikawau\" folder to my raster mode and it will run through.