When running an application on OS X, like Mac Imager CLI, you receive the error "Your security preferences allow installation of only apps from the Mac App Store and identified developers."
- Find the application in the Finder
- Hold down the "control" key and click on the app icon
- Choose "Open" from the contextual menu that appears
- When prompted with the message "AppName is from an unidentified developer. Are you sure you want to open it?" choose "Open"
- The application will now have a permanent exception on that computer and can be opened normally
As of OS X 10.8 (Mountain Lion) Apple changed the default security settings to only allow installation of apps from the Mac App Store and identified developers unless you grant an exception with the above steps.