I am trying to remove the Alternator on my Nissan Sentra 2001. I believe I need to take the alternator belt off. It's tightly on the pulley - see picture 
How to make the belt loose? I am told that there should be a tensioner bolt head and it has a very long thread which screws into the backside of the center bolt/nut that is 90 degrees of the center bolt/nut but I cannot find it.
Is it in this picture?
Try to get hold of a workshop manual; failing that, a Chiltons or Haynes manual for that car. I've just checked the Haynes, and it does show the Alternator replacement in some detail.
A couple of major hints; firstly, you get much better access going in through the passenger wheel-well. Take the wheel off, and the plastic shield between the wheel-well and the engine compartment, and the alternator will be staring you in the face.
Secondly, the alternator sits on top of the A/C Compressor. You'll think you can work around it, but you can't. It has to be moved, so you can get to the alternator bolt correctly. Undo the four compressor bolts, and move the compressor out of the way - do NOT disconnect any lines to the compressor, or you'll lose all refrigerant, the ozone layer will shrink a little, and somewhere, a small kitten will die (OK, not sure about the kitten).
Based off this diagram, the adjusting bolt is not present in the pictures you provided:
It looks like your Sentra has air-conditioning, which means that your belt tensioning mechanism sits above the power steering pump (leftmost belt-driven component).
Different year same engine.
Your image
Loosen the idler pulley nut. And per the posted image loosen the adjuster to remove the belt.
You don't have to remove the AC compressor to R&R the alternator on the 1.8. You can remove the fan shroud to access the lower bolt on the alternator and pull the alternator through the top.
Yes the tensioner is to the left of the alternator - in the pictures above. Slightly loosen the main bolt on the tensioner - not too much. This will allow you to loosen the adjustment rod BEHIND the tensioner, which creates slack and loosens the belt. You should be able to see it - it will look like a threaded rod. Probably a good time to replace the belt and the tensioner, while you are in there... I know that is normal procedure on most vehicles... and the oil leak if possible. Oil will degrade the belt over time.
http://www.instructables.com/id/Adjusting-Nissan-Sentra-Serpentine-Belts-1/
