I stumbled upon this dilemma when I was trying to create a pydev google app engine project in the AptanaStudio 3 edi, to complete the wizard I had to set my google app engine directory.

After a bit of searching, I realized that the "google_appengine" directory was not placed in public on my harddrive, but hidden inside the GoogleAppEngineLauncher.app package (right klick on the application - choose show content).

So, if you run GoogleAppEngineLauncher on your mac, the path to your google app engine root is this: /Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/