i had this problem a couple of days ago and it wasnt easy getting it sorted.. heres what i had to do as told by them
1. Report problem in itunes purchase history - didnt work as was downloaded to long ago the 1st time.
2. buy the update again and they will refund me for it - didnt work as your only allowed to download it once with each account apparently.
3. i was then given this link to try it this way
http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=271872604&s=143441 - didnt work as it takes you to same place as itunes store page
4. They allowed my account to re download everything that i have bought including all music, software update, tv shows etc - this did work but took some time to get everything!
i was emailing one person at apple throughout the whole thing and she was very helpful.
try emailing iTunes Store <iTunesStoreSupport@apple.com>
This is the last email that they sent me...
Dear ......,
I understand this page gives you the same prompt you've been receiving. I know this must be very frustrating, and I have a fix for this issue right now.
The iTunes Store allows a one-time redownload of all your purchases should you lose them. Since this was just the one item, I thought it would be better to repurchase it for a refund, but since it isn't working I have used it to repost this item to your account.
Since the one-time exception has been used, I strongly recommend backing up all your data. There will be an article with back up instructions later in the email, and there will be information on where to find the file for this software to back up, as well.
Follow these steps to download the available items:
1) Make sure you're using the latest version of iTunes. It can be downloaded free of charge from the iTunes website:
http://www.itunes.com/download
2) Open iTunes.
3) From the Store menu at the top, choose Check for Purchases.
4) Enter your account name and password, then click Check. Your purchases should begin downloading. If you receive an error message while downloading, try again after turning off any firewall or web-accelerator software that you may have installed.
etc....