Very simple DWG import code:

public Result Execute(ExternalCommandData commandData, ref string message, Autodesk.Revit.DB.ElementSet elements)
{
   Document curDoc = commandData.Application.ActiveUIDocument.Document;
   Transaction transaction = new Transaction(curDoc, “Import DWG”);
   transaction.Start();

   DWGImportOptions options = new DWGImportOptions();
   options.Placement = Autodesk.Revit.DB.ImportPlacement.Origin;
   options.OrientToView = true;
   options.View = curDoc.ActiveView;
   Element element = null;
   if (curDoc.Import(@”C:\Temp\Test.dwg”, options, out element))
      transaction.Commit();
   else
      transaction.RollBack();

   return Result.Succeeded;
}

This code only shows very simple way to import a dwg file. The DWGImportOptions contains many properties which you can use and test to get what the best you want.