The top alternator bolt is shown below. To tighten the belt, loosen the large nut on the back of the top bolt. Also loosen the bottom alternator mount bolt, which it sounds like you've already found. Then tighten the smaller nut on the adjuster until you get proper belt tension.

Once you get the belt tight, tighten both the large nuts. At this point I think it is a good idea to back off on the adjuster just slightly to take the stress off it.

I had a heckuva time adjusting my 240D, and even broke the adjuster bolt. Upon complete removal of the alternator, I discovered that the top hole had been ovaled out, allowing the alternator to cock sideways and get stuck. A rebuilt alternator solved that problem. Just a personal experience for you information.

Attached Thumbnails

 

__________________
Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References