Currently Being ModeratedFeb 20, 2013 7:07 PM (in response to wvductman)
Control-click the installer and choose Open.
Currently Being ModeratedFeb 20, 2013 7:38 PM (in response to wvductman)
This is a new security feature in Mountain Lion; if you want a permanent change rather than a one-time allow, go to System Preferences > Security & Privacy > General and change the settings from MAS to one of the other two choices. This is only recommended though if you are sure of the app and the developer.