If you are going to add a wall and base as a part and mate them as part of the assembly, you need to extrude the THICKNESS to 0.1 (to show the surface as a lamina) rather than the length and the width.
If you click on the render icon the screen will refresh with any new materials and scene -...
We tend to use the scene tool quite a lot in school for preparing jpeg views of models for inclusion in dtp documents as part of the coursework required.
Choose each part of the model and apply materials.
click on the scene icon and choose the last tab - scenery.
change from the 'view'...