If I were you Graham I would drop Themza themes like a 'hot potato', as they are a pain.
If you had looked inside the zip before you extracted you would have found a number of .xml files, related to the images located in various mod folders.
If you want to try and strip the theme of these .xml files first, in a zip clean up, then open the zip, and list the files in TYPE order and then highlight and delete the ones whose file extention is .xml.
That done extract the zip to it's theme folder onto your desktop. Then using a Text Editor (Notepad) find and open config.php in the theme's folder.
Somewhere in the config.php will be a reference to the xml files. It's a bit of PHP which links back to Themeza website which is not necessary. So delete that line or lines of code.
The fact your theme isn't showing up in the Theme selector could be caused by the cPanel, if you use such a thing on your website, as Richard suggests.
Mary