I don't think you can directly add KML to ArcMap (I could be wrong here), but you can use the KMLtoLayer tool to make a .lyr file in the user's temp folder and then add that layer.
It sounds like you're code is not running inside of ArcMap but another application. If so, try using the AppROT class (in Esri.ArcGIS.Framework) to find ArcMap (if it's open) or to open ArcMap & add the layer created from the geoprocessing tool.