Assuming you just started up, press "g" twice. Move your cursor over a building, it should display a white box around it.

Click on the building. Press "v" to toggle it on and off. Anything more complicated than that you have to get into the code, but you can do some neat things just by editing the shader (GenerateVolume.c"), saving it, then pressing "r" (refresh shaders) in the program (and then press "c" to clear the chunks and refresh them).

Future iterations have much more complex methods for adding arbitrary geometry, as demoed in the videos.

The green box cutaway:

Hold down ctrl (and keep it held), Move it while holding the left right mouse button to get it at the appropriate z level (up/down) You will likely ALSO need to move it on the XY axis holding the left mouse button just to keep it in view. That probably sounds confusing, but in isometric mode keep in mind that xyz in worldspace all project to another xy axis in screenspace. You can see in some videos how I move the cutaway cube down along the z axis while keeping it in the screen by moving it back up along the ground plane (xy).

At 6m44s in this video you can see how it should function when you move the green cube if it is at the right z level: https://youtu.be/R0ul1674F1k?t=6m44s