You tighten/loosen using the nut at the top of the threaded adjustment rod. If you shine a flashlight in the area in front of the tensioner, you will see a pointer.As you make adjustments the pointer moves either left or right. You need to find out where the adjustment should be for your particular engine. Kind of difficult to verbalize.

Search the forum on Belt Tensioner Adjustment or something like that.

Or you might just swing into an indepent MB shop and have them do the adjustment. Should be a cheap job.

Steve