HI , just wondering and asking:
1. You did set the output in MD8 that is cmd ( and y)
2. With a source selected you switch to MM
3. In MM you have to select Modul8 syphon input . By default you see a checkerboard
Under the Text "Syphon" you see Modul8 - Main View
If you select this you will see the selected clip from MD8.
I hope this helps .....
here is the tutorial from the madmapper website http://www.madmapper.com/modul8-madmapper-tutorial/