Mercedes: heater, apparent leak, climate control

Question
I seem to have trouble getting the heat to activate in my E320 sedan (2004).  I just bought the car pre-owned and everything else (except for an apparent leak over the passenger side overhead hand-grip) is fine but when I try to heat the car, following the directions in the manual, all I get is cold air blowing.  I know the heater works because the dealer had the heat going when we were test driving, and I have had the heat on myself but I can't figure out how.  Do I hit AC off?  Does Auto go on before the temp is set?  I begin it at 72F, per the manual, but it isn't responding.  What is the procedure?

Answer
The AC should be off. The Heat should be on with blower on high and temp control on high. Remember that the climate control is keyed to the water temperature. It is not activated until the heater core water temperature has warmed. This process keeps cold air from being blown over the passengers.
Good Luck
Rob