If you've purchased MoI v2 after May 1, 2014 you can receive a free upgrade to v3 by putting in your v2 license key here:
http://moi3d.com/3.0/upgrade.php

You'll receive a new version 3 license key and download links for the installers right on the page there, please save your license key in your records for future reference.

Thanks,
- Michael