If I understand well, you would like to map the main screen of your computer.
I am not so sure on how you can do this, since if you are using Madmapper on your screen, this is what you will be outputting. Maybe I do not get exactly what you mean.
Anyhow, if you want to map a part of your desktop, you could try syphon virtual screen, which creates an extension of your desktop and allows you to 'syphon' it. Easy to use and seems to work well
http://andreacremaschi.github.io/Syphon-virtual-screen/Best
ALain