Changes introduced to Mac OS X in version 10.11 (“El Capitan”) create some issues with SpaceChem that we’re currently working to fix.
Regardless of your OS X version, you’ll need to install Mono in order to play SpaceChem:
If you are seeing this error on Mac OS X 10.11 or later you will also need to follow these instructions:
https://github.com/leafi/fix-spacechem-os-x
If that doesn’t work or you need assistance with anything else, please contact support@zachtronics.com and we’ll do our best to help.