It would appear that the issue is caused by a code signing problem. A simple workaround is to launch the program from Terminal. Open a new Terminal window and type "/path/to/Pixel Shootout.app/Contents/MacOS/Pixel Shootout", replacing '/path/to' with wherever you saved the application (e.g. 'Downloads'). An example of this command would be:
"Downloads/Pixel Shootout.app/Contents/MacOS/Pixel Shootout"
The quotation marks allow you to have spaces in your command.
I will try to fix this issue as soon as possible.