Your friend is talking about the ACC panel on 1995+ cars. It will show the status of most of the AC sensors and some interesting stuff like road speed, rpm and battery voltage.
What year is your car? On the 1996-2000 C280 with LCD digital "DIN size" ACC panel, just press the "rest" button for five seconds while the key is in position 2.
1998 C230 "Black Betty"