the "select all" and then "shift and select" option but that doesn't work

Shift+click to deselect something from a set of selection. [#-o]

I can't select a surface that is INSIDE a room. when i trie to select it is always the wall of the room that is selected.

By default there is no selectable floor, you need to draw a floor using Create rooms tool. Later by modify rooms section apply the texture.

How do i attach my workfile (sh3d) in this reply?.

Follow the instructions by hansmex to attach a file.
By simply : Upload the file in mediafire.com
Copy file link(file location). Paste in post when you replay. [:-$]