I do not know about a button,
but the function is regulated by this part Y66/1;
http://www.classic-roadster.de/cgi-bin/showitemlocation.cgi?TYPE=W140&X=5&Y=10&SORT=ITEM&START=Y&ITEM=Y66%2F1x1
which you can find again in;
http://mb.braingears.com/W210/w210/trans2_elec.htm
but after futher thinking, the above is perhaps only related to not
engage R and P while driving...
Then, this page should be more related;
http://mb.braingears.com/W210/w210/trans2_shiftinter.htm
So, I think I would have a look under the panel above the brake pedal
and push/release the correct bowden cables.
br,
syljua