Will not install, error can't open because it is from an unidentified developer. "Your security preferences allow installation of only apps from Mac app store & identified developers.
You need to change your security settings in System Preferences.
Change to allow downloads from Anywhere.
Here's a link that will help:
Rather than changing your security settings, you could consider using the right-click method described on the same page under the heading "How to open an app from a unidentified developer and exempt it from Gatekeeper".
If nothing else works, Reunion has been available for the Mac for a long time, and yes it is Lion and Mountain Lion compatible:
Online there is also http://www.myheritage.com/ which I like a lot.
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