how to decorate the outside walls of the houses

In the top navigation bar, click "View" than uncheck "hide wall", then you will be able to see all walls in 3D view

Within the HS program the process is the same as the inside. Either in roam or 3d view, click material & apply or with material brush or click on wall, got to tile flooring and then apply material/draw designs etc. The thing to remember is that with using 'tile flooring' the material should be in your favorites folder- premium materials will not be available.