To setup your screens in ProPresenter, you first need to ensure that your system screens are setup appropriately before you configure the screens in ProPresenter.
To access your system display settings go to "System Settings" which is either located in your dock or by clicking the Apple icon in the top left of your Mac and choosing "System Settings". Then, in system settings, click on "Displays"
Once in Displays you will see your screens listed across the top. So, the first thing you should do in this process is make sure that all the screens you expect to show up as connected to the computer show up here. If not, then you would need to check all cabling and connections until it shows up.
Below that list of screens you will see the option of "Use as". This option is normally set to Mirror by default, but you will want to change this to an "Extended Display". So, you will click the drop down menu that says "Mirror for..." and choose "Stop Mirroring".
Once you choose "Stop Mirroring", it will say "Extended Display"
Now, you can open ProPresenter again (if it was open during this process, restart it) and configure your screens. You can learn more about configuring your screens inside of ProPresenter HERE.