IIRC, the DB lounges at Berlin Hbf and Frankfurt Flughafen Bahnhof have paid T-Mobile hotspot access, so I assume that the Munich Hbf will have the same.

If you don't have another form of web access (such as a broadband aircard), you can buy a 24-hour pass that is good at any T-mobile hotspot for the 24-hour duration. For example, I purchased a 24-hour pass at the Westin Leipzig on a Friday evening and was able to use it at the DB lounge in Berlin the next morning.