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/