It's time to get rid of that grey checkerboard pattern covering the walls and floor. You can apply either a material or a material instance to a surface. We'll discuss the differences later. Both types of materials show up in the generic browser with a green border.
Let's find a material that we can apply to our CSG surfaces. Open the generic browser, and in the filter list, check Material and Material Instance Constant. If you have anything else checked, like Static Mesh, uncheck it.
Click on some packages. Any of them starting with HU_
contain human environment assets, which is a good start. You'll see that a few materials show up, but not many. This is because the packages haven't been fully loaded yet.
Select a package, right-click on it, and select Fully Load. You can select multiple packages at the same time by ctrl+clicking on their names, or by shift+clicking to select a block. Select all of the HU_
packages now, and fully load them...