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Blocking free software is something apple just do. The idea that software can be free and for everyone is not lining up with Apples agenda.
I know that a similar popup in windows has an option hidden way out to the right, where it allow you to install anyway, so first try and scroll out there and see if there is a button that allows you to do that.
I will see if i can get some pointers from those that knows more about Apple -( That is Everyone.. : ) I will post here if i do, and please let us know if you need more help.