Perhaps this will save someone else a couple of hours. Some users of PixInsight may have a minimal understanding of command lines and a few may have none. Mine is pretty minimal and very rusty.
I tried several times to install PI-macosx-x86_64-01.08.03.1123-20141118-c.zip on OS X 10.9.3 Mavericks, sometimes by extracting it in Zipeg and saving the extraction files into /Applications, other times by saving the zip file first to the Downloads folder and then extracting, or by saving to /Applications and then extracting. I tried rebooting after extracting and waiting overnight to ensure that the download completes. The result was always the same, a small window with the message:
“The Pixinsight Core application has been launched from a directory that does not meet the requirements of a valid PixInsight distribution.”
I noticed that Gerry Doyle in Reply #18 on this thread said: “I'd used a 'Zipeg' app to unzip the download - this time round I used the embedded OS X utility and it unzipped properly.” I noticed that J.C. supported this method. I didn’t know what utility Gerry was referring to. The only relevant one that appears in my utilities folder is Zipeg. So I guessed he must have referred to the unix utility unzip or maybe zcat. Following an OS X manual, I opened the terminal program and tried zcat and failed to get results. I browsed online and found a recommendation to type “unzip” into the terminal and then drag the .zip file to the terminal window, drop it on the line, and hit return. This produced the desired results. I located the new PixInsight folder, dragged it to Applications, dropped it in, and as I type this, fifteen upgrades are downloading from within PixInsight.