Duet should open and ask you to install a graphics driver and restart your computer.
Please login to an Admin account to install
Make sure System Preferences > Security & Privacy setting Allow apps downloaded from "App Store and Identified Developers" is enabled ("Anywhere" also works)
If those don't work then please try to drag the Duet app file to Applications and then try opening again.
If for any reason these steps don't solve the problem, please contact us at firstname.lastname@example.org – we'd be happy to help out!