Look for the right mailing list in the mailinglist overview of the Computer Science Department and click it. Enter the email address which your email client uses as “Sender”. For example the Registration page of the ALP mailing list

Probably you registered with another adress than the one your email client fills in into the “Sender” field. Both addresses have to be textually identical since aliases will not be resolved. For example xx@cs.uni-bonn.de =/= xx@informatik.uni-bonn.de. Solution: Register again with the address used by your email client or configure your email client to use as sender the address you registered in the mailing list.

Because all teaching-related mailing lists are emptied at the start of each semester. If you register before that date, your registration will be lost.