This normally happens on some older version of Windows system.
The ZIP file you downloaded is actually the IPA file.
You have three options to fix this issue:
1. You can simply change the file extension to .ipa, then install with iFunBox
2. You can contact our support to get another download link, which you will download a ZIP file that contains an IPA file.
3. After you finish your activation, below the download button, there is an alternative download link for you too.