I just made a booking in "Motel One" http://www2.motelone.com/de/
at Alexander Platz, close to the event as well....
I was hoping to stay in an apartment but it seems too complicated all that paper work, Hotel and Motels is easier and prompt...I am just to crazy busy these days, the last time I went to Berlin I had no kids, so I had nothing better to do then looking around and booked an apartment from a Lady which was so cool and cheap!