Leaking cap: look for "white" deposits on the coolant tank.

Leaking head-gasket: look for oil in the coolant (check the coolant tank).

It might be easier to find the MB Coolant, but the Valvoline G-05 is MB approved (same stuff as MB Coolant) and my local Autozone carries it.

http://www.valvoline.com/pages/products/product_detail.asp?product=10

http://www.imcool.com/articles/antifreeze-coolant/G05-Glysantin.htm

:-) neil