The tensioner is on the very front of the cylinder head, well-hidden by the AC compressor and its bracket. In fact, there is a hole in the bracket for access. You will need to remove the compressor. There is a plug that covers the actual tensioner which requires a 17mm hex key to remove. There is an o-ring that seals the plug to the head, and my bet is that is what is leaking.
The cam box is a pain because you need to remove the cam sprockets, rocker arms, guide rail, and the tensioner to get it off the head. And it's heavy! I can't think of any reason why it would leak unless the bolts holding it to the head are not tight.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe