It's easier to work with an asset in this specific form. Not everything needs to be modeled procedurally in CityEngine.
Check out tutorial 7 (video & link to actual tutorial files):
It's best that such assets are modeled with the +Z axis being 'in front'. Use e.g. SketchUp or Maya or Max to model such assets.
Let me know if you need more details !
with .obj assets, the .mtl file is also needed, because it stores the actual texture reference per material in your asset.
so if you only copy the .obj file, the texture cannot be displayed, even if it's sitting there.
=> check out the .mtl files.
note that you can preview .obj assets in the Navigator within CityEngine ( check Help ). If the texture is shown there, the .mtl is ok. If it shows an 'unknown texture' standin texture, most probably the texture paths in the .mtl are slightly wrong.
(open a .mtl in a text editor to edit paths )