Mercedes Benz repair: wont start when cold, mercedes 300d, valve adjustment

Question
1975, Mercedes, 300D, 120000 Fort Worth TX
I have just acquired this car and I just tries to start and won't actually start when the weather is cold 40 degrees or less.  Also where is the block warmer located, I have a new plug but can't find where to plug it into the block.

Answer
Answer:
Block heater is on the starter side center of engine, if it has one.
Bad glow plugs and valves out of adjustment will prevent starting on cold days.

Cold weather starting links
http://www.mercedesshop.com/shopforum/showthread.php?p=1018529

This is the best link I have, you have a OM616, however the procedure is basic.
Valve adjustment OM617 FYI.
http://www.mercedesshop.com/shopforum/showthread.php?t=107729