The iPhone Dev-Team have confirmed that the latest version of Redsn0w is fully compatible with Apple’s new OS X Mountain Lion software. However, until it has been signed with a Developer ID, you’ll have to bypass Gatekeeper to use it.
In an update on their blog, the iPhone Dev-Team confirmed that Redsn0w has no issues under OS X 10.8. They also suggest how you can bypass Gatekeeper to get it running.
Update #1 7/25/12: redsn0w is compatible with today’s retail release of Mountain Lion OS X 10.8. Until we start using an official developer ID for it (!), you’ll need to use the new Ctrl-Click-Open security bypass the first time you run it after downloading.
If you’re unfamiliar with Gatekeeper, it’s a new feature that debuts in Mountain Lion that’s designed to stop malicious software being installed on your system. By default, it only allows you to open apps that have been downloaded from the Mac App Store, or from an “identified developer.”
Until Redsn0w is updated, Gatekeeper won’t let you run it. You can either get around this using the method suggested by the iPhone Dev-Team, or you can disable Gatekeeper within the Security & Privacy menu of System Preferences. Of course, by doing the latter you are opening your Mac up to malicious software, so you’ll need to be sure everything you download is safe before you run it.
Source: iPhone Dev-Team