You can create a new TIN surface from a binary .tin file using the TinSurface.CreateFromTin() method. This method takes two arguments, the database for the drawing to which the TIN surface will be added, and the path to a .tin file, as a string.
[CommandMethod("CreateFromTIN")] public void CreateFromTin() { using (Transaction ts = Application.DocumentManager.MdiActiveDocument.Database.TransactionManager.StartTransaction()) { // Example TIN surface from Civil Tutorials: string tinFile = @"C:\Program Files\Autodesk\AutoCAD Civil 3D 2013\Help\Civil Tutorials\Corridor surface.tin"; try { Database db = Application.DocumentManager.MdiActiveDocument.Database; ObjectId tinSurfaceId = TinSurface.CreateFromTin(db, tinFile); editor.WriteMessage("Import succeeded: {0} \n {1}", tinSurfaceId.ToString(), db.Filename); } catch (System.Exception e) { // handle bad file path editor.WriteMessage("Import failed: {0}", e.Message); } // commit the transaction ts.Commit(); } }