There are numerous articles here about replacing the infamous "U-Gasket" as some call it.
The official MB task number on the CD-Rom is 01-212 - Removing and installing top front cover.
This also requires that you get the serpentine belt off and that requires 2 special tools or some fabrication/ingenuity on your part. Performance Products sells both and I suspect this site may as well.
The following thread contains some good advise from Larry Bible and another poster further down gives the sealant number. You can get it at your MB dealer. I would describe the task your about to do to the MB parts counter person because there are many different sealants mentioned in the MB repair manual(s).
M103 - Upper Timing Chain Cover and Cam Seal Leaks