On a Mac this is usually caused by an issue with your unpackaging software (to uncompress ZIP files). If you have multiple uncompress applications (i.e. The Unarchiver) you should try one of them.
This is caused by the download being cut off mid-session of download(usually with Internet Explorer), leaving you with only part of the file. There are a couple ways to try to resolve this quickly:
- Re-download using a different internet browser (i.e. Firefox)
- If the first suggestion doesn't work, clear you temporary internet files by selecting "Tools > Internet Options > Delete Temporary Internet Files" in your internet browser window.
- If neither of the above work, try downloading from a different location with a stronger internet signal.