By default, Microsoft IIS only supports a small number of files. This is what you need to support KML and KMZ files in IIS.

The MIME type details are:

Extension: .kml
MIME type: application/vnd.google-earth.kml+xml

Extension: .kmz
MIME type: application/vnd.google-earth.kmz

If you need to know what to do with these mime types, I have a step by step guide for setting up RSS files at /index.php/2009/06/supporting-rss-files-in-iis/ – Just use the different extensions and mime types.