some issues and questions regarding your requirements. I am not sure how familiar you are with Mac's and Mac security. In order to do this, you the user installing the software must be "authenticated". So if you had some idea that a "silent" install could be run, it cannot. Also, there is no "run" or RunOnce" registry key to automatically start an app on system startup. This is a complex and security driven procedure. And with each new version of OS X, it becomes more difficult. Say with Snow Leopard, it is relatively straightforward. With Lion, it became more difficult and with Mountain Lion, it is very difficult without proper authentication.
So What I would need to know, is what version is this for.
What are your intentions as far as "user interaction" durning the install, and any other details you might have.
Mark