"Arduino is damaged"

I have a Macbook Pro with Mavericks installed, I've tried to download the 1.0.5 IDE for Arduino Uno and I've extracted it with alZip. But when I try to open the application it says that it's damaged and I have to delete the file. I've tried to delete it and download it again several times and I've just downloaded Java. What could I try now?

Works fine for me. Perhaps alZip is doing something wrong. Try a different utility for un-zipping. The App Store lists "The Unarchiver" and "iZip Unarchiver" in the top free utilities list.

Nothing is damaged, it's because of Gatekeeper.

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