I've downloaded the Arduino APP at least a dozen times and every time OSX says that it's corrupt. So... I downloaded the Arduino source from Github with the intention of using the Apple App developer to compile it. But... I had to download XCode from Apple... and now I'm stuck. I can't figure out how to create the Arduino.app so I can start learning more about the Arduino.
Did you download http://arduino.googlecode.com/files/arduino-1.0.5-macosx.zip, pointed to by http://arduino.cc/en/Main/Software ?
Did you un-ZIP it successfully?
Did you copy the resulting Arduino.app file to the Applications folder?
Is this version 1.0.5?
See this thread:
Updated version of 1.0.5 for OSX has been uploaded.
Should fix the app signing issue.