Ah, excellent - thanks for telling us. It is very likely that the same issue will come up again in the forum. Seems an odd way for Kobo to distribute the file, but there you go.
Looking up 'kobo .zip' on google shows you are not the first to have the issue. If you get any more Kobo oddities, you may find some help at http://goodereader.com/forum/showthread.php/3946-The-Ultimate-Kobo-Tip s-amp-Tricks-Thread