I don't know the version you have, but in older versions you specify the dielectric layers in the basic parameter window.
Try to locate basic parameters in the menu. It will open a new window. It normaly shows two layers, a conductor layer and one top layer that is air dielectric up to almost infinity.
When you want to add another layer, you just click on the new layer icon (right part of the screen, very close to the red cross) and a new window opens. There you specify the top height of the layer and the substrate properties. If you have another layer on top of the previous, just add a new layer and just mention the top height. You don't need to mention thickness, as the program calculates that based on the top height of the layer below.
Before specyfing anything, set the units (mm, m, inch, etc). You also specify the grid size and conductor types in the basic parameter window.