Follow these instructions:
- Right-click (ctrl+click) the KSP.app file and select the ‘Show package contents’ option
- Navigate to the Contents->MacOS folder, inside it you should find a Kerbal Space Program exec file
- Open the Terminal and type ‘sudo chmod +x’ (without the quotes, note the blank space at the end) and drag and drop the exec file from the previous point from the Finder to the Terminal, and then hit enter.
- When The Terminal asks for your password, use your Mac password, not the KSP one. It will seem as though you were not typing, but you are. Hit enter after you finish typing.
If the Terminal doesn’t give an error message, you should be able to just double click KSP.app from the Finder to start the game.
If you're still having issues, submit a support ticket here.