At first I thought you were just missing the appropriate area to do the change from 12 to 24 hour time in the intrument cluster. But now I believe that the change from 12 to 24 is done in the cluster ONLY on cars without COMAND.
On your car, I believe the change is made in the COMAND unit, not the intrument cluster.
I believe you need to push the "system" button on the COMAND unit, and follow the path where the time zone change is made. It may be useful to check out the COMAND operating book and check out the intructions there.