Are you very sure an “Export” menu existed? There has been a File → Save menu where you can save screenshots (as raster graphics) or placemarks and Keyhole markup (as KML/KMZ files).
What are you trying to achieve? Basic geometries in Google Earth are colored KML lines/linestrings and polygons.
Do you want to save changes you made to a KML file that you imported before?
Do you instead mean 3D models that are part of the 3D buildings layer (not imported)? These are property of Google and neither available nor licensed other than for viewing purposes.