Personally I like to take the loaded STL and TypeCast/Clone it into an entity. Since entities are referenced passed this is a safe way to create a new entity and get your STL loaded into it. I tossed a few things in there to make it easy to view in the eyshot viewport as well. Regenerating the entites, and invalidating the viewport are key to seeing your entity after it has been added. Just make sure you already have a viewport. :)
string Path_STL = "C:\\Users\\bdg\\Documents\\TestSTL.stl";
try
{
var currSTL = new ReadSTL(Path_STL);
currSTL.DoWork();
Entity ent = (Entity)currSTL.Entities[0].Clone();
ent.ColorMethod = colorMethodType.byLayer;
ent.LayerName = "TestLayer";
ent.EntityData = "TestSTL";
viewportLayout1.Entities.Add(ent);
viewportLayout1.Layers[ent.LayerName].Color = Color.Aquamarine;
viewportLayout1.Entities.Regen();
viewportLayout1.Invalidate();
viewportLayout1.ZoomFit();
}
catch (Exception ex)
{
Console.WriteLine("Load STL Failed: " + ex);
}