The tool you are talking about if I'm not mistaken is here.
http://www.peachparts.com/shopforum/showthread.php?t=126728
It's been a while since I've done this job but as I recall you loosen a bolt and then use the special tool to hold the pulley where you want it, i.e., to tension belt and then tighten the bolt.
Edited:
Or you could buy the tool here
http://www.samstagsales.com/mercedes.htm it's $57.95