While I'm a Windows user rather than Mac, from the error message it appears the fzz (which is a renamed .zip file) is either missing or corrupted. Things to try: Check permissions and owner ship of the file (don't know how in the gui on a Mac though , if you are comfortable with the Unix command line
ls -l /Applications/Fritzing/JB Fritzing projects
would do it) If the file is there, check for zero length (in which case you may have or had a full disk which will truncate the file when it is written). If the file is there and has an appropriate size, rename it to Bendo Box v5 5cm.zip and try and open it with a zip utility (again don't know the name on a Mac, I use 7zip which is open source and may also support Macs on Windows). Hopefully that will give you a better error message than Fritzing is and thus more of a clue as to what is wrong. Even better, perhaps someone familiar with Macs will give you some better suggestions . In any case good luck! Backing up valuable files is always a good bet, but now is a little late to start if you haven't been (but it is a good suggestion for the future in any case).