I know this is old, but I thought I'd reply just in case you hadn't found this, or, someone else has the same problem.
Have you tried the "Open With" plugin? When you install it, configure it to use your zip program when you right click the book you are trying to unzip within Calibre. It won't load the book into Calibre, but it will let you read it on your computer.
If you want the unzipped version of a book loaded into calibre, you have to unzip it first. Be aware, Calibre usually expects 1 file 1 book, so if you have a multiple files within the zip file (say a bunch of HTML files that make up a book), you are best to convert those files to an epub before import into Calibre, otherwise, leave them as a zip file.
I myself have zip files loaded into my library. They are books that have been scanned into multi jpeg files, and placed in a zip file. If I need to look at the book I use Open With plugin to unzip into a temp location, read what I need, and then delete the folder of unzipped jpegs.
Anyway food for thought in case someone else has the same issue.
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