The "Static Objects" tab/option/whatever you call it is only for models. What you find in the floor folder however is only textures.
To add a floor with the desired texture instead use the "primitives" tab which currently only has the ability to add a plane. You will see that there is an option to pick a material, click it and navigate to the floor folder for mansionbase and you will see the floor there, now just click it.
Now that you have selected a material you click and drag on the grid to add a floor.
After the floor is added you can adjust the tiling/mapping or select a new material for it if you regret your choice.