Hi Peter, there's a recent thread discussing this here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=8159.1

The first thing to try is does it work ok if you move the app to your /Applications folder and run it from there?

If still not working, can you check if you have write permissions to the Info.plist file? To find the file right click the MoI app and choose "Show package contents" then inside there go to Contents and you'll find the Info.plist file, right click that and choose "Get info" and in the info pane go down to the bottom section named "Sharing & Permissions", if you expand that section does it say you have both read and write access to the file or does it say something like read only? Maybe you will need to take ownership of this file in order for it to be modified, I don't know yet why that's different since Sierra though.

Something has changed with file permissions in Sierra that is causing this problem but not for everyone it seems.

- Michael