Hi Terry, yes the 3.1 upgrade is to support running on Sierra where the original v3.0 doesn't work. If you already have v3, you can get the v3.1 upgrade for free by putting in your license key here: http://moi3d.com/reinstall.htm . Just make sure you're putting in your V3 license key there - if it's giving you v2.52 on that page it's probably because you're putting in a V2 license key. The V2 license key will start with "VER2", make sure you're using the one that starts with "VER3".

- Michael