The rooms are called "saal" in german. Check the schedule to figure out which talk you'd like to watch and in which "saal" it's located and then pick the corresponding stream.
The schedule for today is at http://events.ccc.de/congress/2011/Fahrplan/day_2011-12-27.e...
Unfortunately, Cory Doctorow's talk just ended. :-) But as every year, recordings will be made available later on (probably in a week or so).
Enjoy!