Gatekeeper is a new security mechanism added to Mac OS X Mountain Lion. Gatekeeper is intended to prevent applications from unknown sources being installed without your knowledge. To work, this new software requires that all applications are signed with a Developer ID provided by Apple. Adobe has been working with Apple and is signing all future applications, including the CS6 products. However, older products released before this new feature (for example, all CS5 and CS5.5 products) have not been signed. Therefore, a Gatekeeper warning message appears when you try to install them on systems running Mountain Lion.
If you'd like to verify the digital signature on older, pre-Gatekeeper Adobe installers, follow the procedures below.