If you want to make very simple buildings, open blender and resize the basic cube to your building size, then export it to .obj.
Open that file using the "files" menu at the bottom of your construct 2 project, and point a Q3d model in game to use that model.
Finally, get a texture of a building that is just a flat view of it, and put it in as an animation. Name the animation "DiffuseMap" exactly.