If you want to add a 3D object, meaning furniture, fixtures, accessories, etc. to your project, first open the Components Window (on the left side of the screen) and select the Furniture tab:
Search for the object you want to add, click on it and keep the mouse (or your finger) pressed down, dragging it and then dropping it into the room. When you release the object, it will drop into the room.
At this point, the object is still selected (you'll see it outlined in green), so you can keep moving it around the room and place it in the desired position. You can move, rotate, delete, or otherwise interact with an object only if it has first been selected.
If you move the object towards a wall, the Application will automatically change the orientation for you (click here to learn how to disable this feature). If you want to rotate the object, use the arrows circling the object: