I'll take a look and see if we can get this fixed in an upcoming release. Unfortunately, Google Earth is exporting KML files that aren't using the standard KML namespace so until BaseCamp develops a solution to handle this you can do the following.
1. Open doc.kml from within the KMZ bundle. (kmz files are really just zip files, so change the "kmz" extension to "zip" and extract it). 2. Change line 2 of doc.kml to <kml xmlns="www.opengis.net/.../2.2"> 3. Save the file and drag it into BaseCamp.