To select multiple elements, press the Shift key, hold and click on the required elements to select them. In the properties panel that opens up on the left, go to the Common Properties of All Selections menu, where you will see one or more drop-down menus, each one containing the editable properties of each element category within the selection (eg. 1 common drop-down menu for selected Walls, 1 common drop-down menu for selected Floors, etc).

Under the Common Properties of All Selections header, select the drop-down menu named after the category of elements you wish to assign a common material to. When that menu opens, go to the corresponding material menu (naming of these options differ depending on between categories; for example it may differ in the Wall Section menu vs the Tilings menu.)