Someone just asked about this, so sharing the same information here in case there are other takers.There is no indication that the concert is sold out but online ticket sales have stopped. You will actually have to go to the ticket outlet, KOKA36, to buy them directly.
Here is there information:
KOKA36 Konzertkasse
Oranienstr 29 <---- click for map
(030) 611 01 313
Mo-Fr: 9 - 19 Uhr
Sa: 10 - 14 Uhr
If your German is no good, just give them a piece of paper with "Iron & Wine, 20:00 Uhr, Mittwoch, Passionskirche" on it.

There are 3 of us currently going.
Cheers!