Another thing you can do is remove the com.apple.quarantine extended attribute that Safari puts on downloaded files and which is what triggers the "Gatekeeper Path Randomization" mechanism that makes the app read-only and makes MoI have that error.

There's some info on how to remove com.apple.quarantine here:
https://derflounder.wordpress.com/2012/11/20/clearing-the-quarantine-extended-attribute-from-downloaded-applications/

If you have MoI in /Applications, the terminal command to do this would be:

sudo xattr -r -d com.apple.quarantine /Applications/MoI\ v3.1.app/


You'll need to enter in your admin password. If you copy the app over into /Applications that's supposed to be automatically removed but it seems that it isn't happening for your case. Are you maybe using a different method to copy the file, or copying the .zip file over first instead of the unzipped app? Whatever the reason hopefully manually removing that attribute will do the trick.

- Michael