If you’re getting errors when attempting to launch Minecraft and you’re running Mac OS X 10.8, it’s probably due to the new Gatekeeper feature. Here’s how to fix it without disabling Gatekeeper.
Open System Preferences, and click on the “Security & Privacy” icon:
Click the lock icon in the bottom-left corner of the window to allow changes.
In the lower area of the window, there are options for “Allow applications downloaded from:”. Click the “Anywhere” button.
Launch Minecraft. Then Quit Minecraft, and go back to the System Preferences window change the “Allow applications downloaded from:” setting back to what it was previously, and Minecraft should launch fine from now on.
In general, you should be able to right-click an application that doesn’t want to launch, to bypass Gatekeeper (and should only have to do this once). For some reason that didn’t work with Minecraft, but this method did the trick.
I was glad I didn’t have to turn off Gatekeeper completely, it’s always nice yo have extra security on the Mac.