On a Mac, navigate to the location of the Zip file in Finder. Double-click on the Zip file. Archive Utility will launch automatically and create a folder with the contents of the zip, within the folder containing the original zip.
On a PC, find the Zip file you want to unzip, Right-click on it, and select "Extract All". In the "Select a File and Destination" window, click "browse" to choose where the unzipped files should be stored (by default, Windows unzips compressed files to their original location). Finally, click "Extract" to unzip the files.
NOTE: The built in software for unzipping files on your computer may be more likely to report an error in zip files where it perceives there to be an issue. While we can't recommend third party software, it may be worth noting that some other software will be more likely to push past these perceived errors and get to your content.
If you're still having trouble, please contact us so we can reinstate the download on any purchases you wish, this will allow you to try the download again.