Go to https://rad1o.badge.events.ccc.de/challenges/ to register and see the list of challenges.
The download file for the first challenge is here: https://rad1o.badge.events.ccc.de/_media/code.b1n
The GSM challenge might be buggy. If you are unsure, just hold off on the answer.
The Challenge closes on Day 4, 21:00 at the muCCC village. Be there if you participate.