Hi Matthew, I'm assuming your talking about ArcGis Online, but correct me if I'm wrong. Generally, you don't export a whole map, but a particular feature layer. In your case, I'm guessing that is some polygon features. According to this document, Use hosted web layers—ArcGIS Online Help | ArcGIS, you can't export as a KML, but you can export as a CSV, as long as the features are downloadable, which will enable you to open it as a spreadsheet. If you have ArcGIS Desktop, you can also export to a shapefile, and then convert the shapefile to a KML in Desktop.