Hi,
I have not used momentum for years now but i remember having similar issues at that time.
I solved the problem by loading substrate definition example of GaAs substrate for instance or PCB materials, in order to see how the layer where mapped. The documentation of momentum is also well done.
From what I remember, you have first to define the dielectric layers, and then in the next tab, you have to map metal layers in between the dielectric layers.
The layer 5 will be define as a close boundary if I'm right.
For the metal layers, you have to define if the geometry you need to draw is a slot or a strip. For a CPW line for instance, you can either choose to draw the center strip and the lateral ground planes strips or you can just draw the slot that will be substracted from a ground plane.
The most straightforward for beginners is to set all metal layers as strip.
Finally, in between metal layers, you can define via connections.
I hope it will help you !
reegards,