I have just changed the System Requirements page to reflect the fact that we have not tested PixInsight 1.8.0 on Mac OS X 10.6 and 10.7.
For now, I will upload installation archives for the previous version 1.8.0 RC7, but I am unable to build a final 1.8.0 version compatible with Mac OS X 10.7 right now. Active development of the PixInsight Core application will continue in January. I am sorry about this, but I do what I can.
On the other hand, why don't you upgrade your computer to OS X 10.8 or 10.9?