Hey Tata, I have to admit it's not a short way from Karlshorst. I attached a screenshot of how to get there. For a different time see www.bvg.de, type Karlshorst in the first field and Kinkeplatz in the second.
From there it is really close to walk.
The adress of the bamf is Askanierring 106, 13587 Berlin.
It is open from 8am - 3pm. Mo - Tue, 8am - 2pm Friday.