This would be a good time to stop at your local MB mechanic, and pay them to try tightening the belt. The hassel factor in changing out a belt tensioner is fairly high. You also need to know how to set the pointer. There are a lot of mechanicals packed in a small space. An experienced mechanic can do the job fairly quickly because he knows the exact sequence of events and knows the shortcuts. This is a critical component.

If you do try to tighten it, remember that there is a large, 17mm I think, bolt that runs front to back down in front of the tensioner. This must be loosened prior to attempting to adjust the tension. If you fail to loosen it, you will bend the adjuster rod.

I would recommend paying someone to tackle this project. In addition, once you buy the part and discover that installation is beyond either your ability or toolset, you may discover that your mechanic refuses to install a customer provided part. A good chunk of their margin is based on marking up parts.

Steve