How to create a new feature layer using ArcObjects
IApplication app;
IMxDocument mxDoc;
IMap map;
IFeatureLayer fl;
// get the handle of the focus map
app = ArcMap.Application;
mxDoc = (IMxDocument)app.Document;
map = mxDoc.FocusMap;
// create a feature layer
fl = new FeatureLayerClass();
fl.Name = yourFeatureClass.AliasName;
fl.FeatureClass = yourFeatureClass;
// add the feature layer to the focus map
map.AddLayer(fl);