This might sound like a stupid question but did you map the texture to the building whilst using your chosen 3D mesh creator, blender, 3dcanvas, 3ds-max, gmax etc, before exporting it to the required format? Bye the way, which modelling tool are you using? I use 3Dcanvas but the principals are the same.
If you're really stuck I could have a look at it for you but you would get a lot more from it if you managed to solve the problem youself.
best regards, Tony