Files?

I just bought a simple game called: 'Tower! 3D Pro' were you are a airtraffic controller (it is powered by the Unity engine, so that's why I'm here...).
But I thought they could have some more buildings in there maps. So, I thought it would be a great idea to make some extra buildings with Sketchup. But after searching the internet for 'Sketchup/Unity' info I still missed some information before I could start adding buildings to the already existing map. So I was wondering how you can open the file from the game that has the game world in it so I can put some extra buildings in the game.

Question:
1. How do you find the right file in the gamefolder to see the game world in the 'Unity Personal' program?
2. How do you open the file correctly?

The pictures contain some folders/files in the gamefolder.