A couple of possible solutions for you:
First, try unarchiving the file with Stuffit Expander. You probably already have it on your Mac and a simple spotlight search should reveal the location. If you don't, you can download it for free from the Stuffit Website.
If that doesn't work for you, try extracting it via Terminal. While it may seem a bit scary, some of the basic commands are quite simple, and incredibly useful. First, open the Terminal app, located in the Utilities folder inside your Applications Folder. Now, type the command "unzip" (no quotes) followed by a space, then drag the offending zip file over and drop it onto the terminal window, this will insert the path for the file. Press Enter and the archive should unzip to your home folder.
If neither of those options work for you, there are several programs out there that claim to be "Zip Archive repair tools". A quick google search yields several results. Your milage may vary, both in terms of cost and effectiveness though. I have no real experience with any of these utilities so can't speak to their effectiveness.