If you look at the tensioner, there is a big bolt at the center of it facing the radiator. I think it's a 17 mm head. Loosen that bolt first. Use a breaker bar or a piece of pipe over 1/2 inch ratchet. Don't take it out. Just break it loose. The adjuster is a long piece attached to the tensioner on the back. Fit a 13 mm socket over it and loosen (CCW). This will cause the tensioner to move. Put you new belt on and then adjust with the same adjuster (CW). There is a plastic pointer on the tensioner that moves when you turn the adjuster. Adjust to about 75% of the full mark or 7 on a scale of 10. When your done, tignten that big 17 mm bolt.
Good luck.
Meza

PS. E-mail me if you need a picture of any component you're not familiar with in my description above. My procedure is based on a 2.6L 190e and Fastlane shows a similar tensioner for your car like the one in 190e, so I believe the procedure is the same for your car.

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Meza
1993 190E 2.6
Advanced Diesel Systems Test and Research Engineer