The issue is within Apple's security policy... You'll need to change your security settings in order to allow OS X open files from "qualified developers" (not sure if it's written this way, my OS is in Portuguese). Anyway, it's on the 'General' tab at 'Security and Privacy' settings.
Now, back on topic: the measurement tool is definitely something I'm looking after as well (my entire professional life is entrenched on CAD software; 16 years on this industry), so any tool that remotely resembles precision drawing is a plus for me. :-)
Naturally, I'm not expecting that a 50$ app would be