Here is a workflow I use to create KMZ files from Civil 3D or Map 3D. My most common need is to create a KMZ of a roadway or railway alignment.
I start with an AutoCAD polyline of the alignment and open the drawing in either Map 3D or Civil 3D (Civil 3D includes the necessary Map 3D command).
Make sure your Civil 3D (or Map 3D) units and zone has a geocoordinate set and that the alignment is drawn at the correct coordinate location. To set the geocoordinate: On the Civil 3D Toolspace, make the Settings tab current and right-click on the drawing name. In the resulting pop-up menu, select "Edit Drawing Settings...". On the "Units and Zone" tab, set the correct geocoordinate system for your project location. Alternatively, use the Coordinate Systems tab in the AutoCAD Map Options dialog box.
Use the MapExport command and export the alignment polyline as a SHP file.
In Google Earth Pro, use File / Import to import your SHP file.
In Google Earth Pro, use File / Save Place As and create your KMZ file.
A couple of things to note: Google Earth Pro is now free to use for commercial purposes and is a better choice that Google Earth. In Google, search "Google Earth Pro Free" to find the link to get your free license from Google.
In the above workflow, make sure your Google Earth sidebar is visible. If not, turn it on at Tools / Sidebar. With the sidebar visible, expand Places. Make sure the SHP place you imported has the checkbox checked for visibility.
Notes from Cadalyst tip reviewer R.K. McSwain: This is an excellent alternative method for users of AutoCAD Map or AutoCAD Civil 3D versions that do not include this functionality. Civil 3D 2015 and later includes an export KML command, and some earlier versions of Civil 3D may include the undocumented command named ExportGoogleEarth.
Regardless of the presence or lack of those built-in commands, since this tip starts by exporting geometry to the ESRI SHP file format, you have the bonus of being able to share these files with ESRI users. (Don't get the ESRI SHP file format confused with the Autodesk SHP file format that is used to create shape and font files. They are different things.)