We have set a new build for Mac OS X as default. This new build is a 64-bit, signed app that should work on Macs that were upgraded to the new Catalina version of the OS.
If you are running an older Mac that does not support 64-bit apps, an older build is available in the following way: go to Library -> Unity of Command -> Properties -> BETA and opt into the beta named "beta".
If you are installing for the first time, and you're asked to enter a product key, CMD-TAB to Steam, then go to:
Library -> Unity of Command -> (right-click) View CD Key
Please copy/paste the keys from here. We apologize for this inconvenience, but this is an older piece of code and we are still working out how to remove this procedure.
https://store.steampowered.com/app/809230/Unity_of_Command_II/