Aircraft Doesn't Collide with Sketchup Model

Hi there,
I am rather new to the world of intermediate-advanced scenery design for FS2004. I'm trying to make scenery for a town named Sentani in Papua, Indonesia (where I live, abroad)

I successfully created Photoscenery with SBuilder and modified the airport AFCAD with ADE and edited nearby coast lines using Coastline Maker. Now I'm trying to do the main task: model the whole town (not THAT large though), building by building.
I have completely no idea about Gmax. So my only "option" right now is to use Google Sketchup, export as a COLLADA, import using ModelConverterX, export again as a FS9 MDL, then compiling a library BGL using Library Creator XML, then placing the buildings using Flight1 Instant Scenery.

I did that successfully and the building is there where it needs to be, but one problem exists: The building is "not solid". What I mean by here is that the airplane can literally fly through the building without colliding.

Any ideas here? I would very much appreciate it if you can help.