Projucer 4.2.4 can't be opened

I’ve recently upgraded to El Capitan (10.11.6) and Xcode 7.3.1 and I’ve been having all sorts of trouble and what seem like spurious errors in Xcode after adding new files to my project in the Projucer. It’s as if Xcode isn’t parsing the newly added files correctly.

Since I was still using Juce 4.1 and Introjucer 4.1 I thought maybe I needed to upgrade - so I upgraded projucer using the built in update check on an older projucer and it downloaded 4.2.4, however 4.2.4 won’t open, with an error message “ can’t be opened”

what is happening here?

Probably El Capitan’s gate keeper. You may need to open it by hand - navigate to the download location, right-click on the Projucer app icon and select open.