MoI licenses are cross platform - you receive both the Windows and the Mac version installers and the same license key can be used with either or both of them.

The upgrade version requires a previous version license key in addition to the v3 license key that you will receive, you can order this version if you already own v2 or v1.

If you've ordered v2 after May 1, 2014 you can get a free v3 upgrade here.