This is an OS error and can be resolved in one of two ways, both detailed below. Option 2 is really intended for IT users as the setting will affect All software, not just Promethean.
Option 1: Override Gatekeeper for only the current installer
Control-Click on the installer. You should see the Finder’s contextual menu.
Select ‘Open’. Mac OS X will display a warning that the installer app is from an unidentified developer and ask the user to confirm that they want to open it.
Click on the ‘Open’ button. This will allow the installer to run on your computer.
Option 2: Override Gatekeeper altogether (For all software)
Go to System Preferences > Security & Privacy
Click the lock icon in the lower-left corner and enter your administrator username and password
Under “Allow applications downloaded from”, select “Anywhere”