Double check that you have the file structure EXACTLY like this. You may have accidentally put everything into a sub-folder inside the original zip.
Code:
voicebank.uar
voicebank_folder_name
install.txt
The install.txt should look like this. You might have mixed up folder and contentsdir.
INI:
type=voiceset
folder=Final Voicebank Folder Name
contentsdir=voicebank_folder_name
description=Description of your voicebank