Typically, the 'fill' should come from the wall itself.
If you are referring to plan view, the fill will be what you'e set as the cut fill in your wall's settings. Then the floor plan cut plane needs to actually cut your wall to show you that fill, but this should happen by default.
If you are referring to Elevation, then you need to have Vectorial Hatching turned on in your Elevation Settings. Then it will show the vectorial hatching of the Material that is applied to the particular side of the wall you are looking at. The hatching applied to that material is the Vectroial Hatching assigned in the Options> Element Atributes> Materials dialog.
Cheers,
Link.
