Hi John,
That usually happens when the user moves the PixInsight.app application bundle after having executed PixInsight. Most Mac applications can be freely moved without problems, but not PixInsight. Once you have run the PixInsight Core application, you shouldn't move the bundle because PI is a modular system. It expects to find some files (modules and scripts) at fixed location on your machine's filesystem.
Fixing this problem is very easy, though. The easiest way is removing the configuration file for PixInsight. With Finder, open your Library folder. Under Library, look for a PixInsight folder (Library/PixInsight). Remove the PixInsight32.plist file (and/or PixInsight64.plist, if you are using both 32-bit and 64-bit versions). This will reset the whole PixInsight settings to the old state (previous to 1.6.9), and everything will work normally again.
In future versions I'll try to make PixInsight more robust against movements of the application bundle.