I also receive the invalid certificate message when the installer opens. My difference is that when I run the installer (gives me no problems with serial number) if fails to finish installing. When the installer reaches the final screen where "Finish" is normally highlighted the "Finish" button is dimmed out and my only choices are to "go back" or "Quit". This is true for both the in-app update & and the updater package I downloaded from macupdate.com.
I doubt my hardware is at issue (somewhat older iMac), however it might be the OS. I'm running 10.7.4 (Lion), rather than 10.8.x (Mountain Lion) which does require properly signed certificates for installers (we're on our own if we install something that doesn't have a certificate).