Problem with exporting models in Unity

I am a beginner in Blender and I need help to export my models in Unity 4.2. I succesfully imported my model, but when I look at it in Unity, I only see one side of the faces, and I keep passing through the floor. Here is the link to my blender file.

Sorry if I have text in French, but it is my first language.
And this is what I see in Unity.

Also, my text, when I export, has the same problem, I only see half of it.

Could someone please tell me what is wrong, and what I can do to fix it?

P.S. I tried extruding all the faces to make prisms, but it took me about an hour and a half for this small
model. I don’ think I would have the time to deal with something like that:

Is there a faster way to do this?

On models, faces only point one direction. This means that from one direction, a given face is visible and solid. From the other side, the face is transparent and objects won’t collide with it. This is true in both Blender and Unity. To see the model properly in Blender, enable “Backface Culling,” found in the Display section of the Properties panel. If you select a face in Edit mode, you can change the direction it faces with W -> Flip Normals

You will have to ensure that every face in your model points the correct direction. Looking at your model, I see that the floor points down, which is why you’re falling through it.

As for walls, it’s the same deal. You can only have faces point one direction. I’d recommend giving your walls some depth, though it would work if you just duplicated every wall and flipped all the normals on the duplicates. If it were me, I’d rather just build the walls again rather than extrude everything.

I guess there is no way to do this fast, but thanks anyway, it works properly now. For the text, can I do the same thing and flip the normals of the letters that don’t show in Unity?

Yes, text should just be a model in the shape of letters, so it works the same way.

Thanks a lot! I can now see every single thing in Unity. It just takes TONS of time to make the adjustments.