The surfaces pane will list the defined surfaces on the imported item. If it is further divided into groups you will need to use the Polygon group editor - set that as the active tool open the Tool settings pane and click the + next to a group you wish to turn into a material. Now in the viewport right-click on the selected polygons and from the menu click Polygon Assignment>Create Surface from selected..., enter the name for the surface you want to create in the box, and click Accept.