Shadow - Unable to detect imported model from sketchup

Hi everyone, I've just started learning grasshopper on my own.

I want to import a model from SketchUp to do shadow analysis. I tried importing as .skp and also .stl.

So, I know that I have to "Brep" the model, but I am unable to select my complex building model. 

I am able to attain my results with simple Sketchup model but not with a complex model.