Mac 10.8 and above introduced a security feature that disables many downloaded apps. Just turn the feature off and try again. See details here:
http://www.baldengineer.com/blog/2012/11/22/how-to-fix-arduino-is-damaged-and-cant-be-opened/
The GUI software is written in Processing, which uses Java. The main advantage of using Processing is that it’s easy to write code, and the application is naturally cross-platform.