Currently the AppStore is bombarded with download requests, which sometimes seems to fail or result in corrupted downloads. The AppStore team is looking into it. We're sorry for the inconvenience.
EDIT: Problem should be solved now. Happy downloading!
I'm experiencing the same problem....
Had to download it again twice, then it was ok
It looks like the download was interrupted half way. These are the checksums of Mendix-7.0.2-Setup.exe: