Material mapping works very similar to Section Profile Mapping in Konstru. Once you download not all materials might be present in your authoring model. Some of them need to be created or mapped to others. If you download a Concrete material from Revit for example it might not be suitable for analysis because Revit materials are very coarse. You might want to map Concrete Caste-in-place to Concrete 6000 psi in ETABS. And this is why you'll see our material mapping dialog quite often on downloading.
On the left hand side you'll see the incoming material. Konstru will let you select a different target or create new in the Target column of the mapping dialog.