Could you post a screenshot of that error message that you are getting from your Mac computer when you are trying to install Premiere Elements 12 Mac?
Are you installing from an installation disc or from installation files for the tryout or a purchased copy?
I am strictly an Elements Windows, but I have seen this type of things happen when I was downloading a tryout from Adobe and I point the download to my Desktop. I had more resources imaginable and still got the message. When I pointed the download to my Documents instead of the Desktop, in seconds, the download proceeded to a successful conclusion.
I am not sure what the Mac counterparts of all that would be.
What is your Mac operating system?
Please view the above and then let us know if any of that applies to your situation.