I think you take the subway from the Hauptbahanhof to Karlstor for the Marienplatz. You could walk too--10 15 min. The locals and the announcer refer to Karlstor as "Stacchus" I think because they don't like Karl whoever he was. There are 2 dept stores in the Marienplatz as well as a small cafe with internet access--free if you eat there.