And just as importantly ... was the disk damaged when you got it (so you are looking to get a replacement for defective goods) or has it happened subsequently and you are wanting to buy a replacement for the damaged disk without having to re-buy the whole thing.
If it is the former, you need to go back to where you bought it with the receipt. If a lot of time has passed since you bought it you might struggle to get a replacement.
If it is the latter, the best way to start would likely be to contact the publisher (WHV I'm afraid), let them know what has happened and ask if they can help.
Example ... I have a copy of The Bundled Doonesbury, which is a 30 year retrospective of the comic series, with over 7000 strips contained on CD-ROM. I broke the CD, and contacted the publisher to ask if it was possible to buy a replacement. Not only did they supply me with a replacement CD, they sent me a full new copy of the book with disk and shipped it from the US to the UK at no charge whatsoever.
Who knows, perhaps WHV can demonstrate some similarly responsive customer service for a change.