I have the model 200, which is a single action version of your 155. Here is a manual to the thing.
http://www.newstyleservice.com/155airbrush/cover.html
http://www.newstyleservice.com/155airbrush/images/bbb155instrubkcovbbb.jpg
http://www.newstyleservice.com/155airbrush/images/bbb155instrubkcovpg1bbb.jpg
http://www.newstyleservice.com/155airbrush/images/bbb155instrubkcovpg3.jpg
http://www.newstyleservice.com/155airbrush/images/bbb155instrubkcovpg4.jpg
http://www.newstyleservice.com/155airbrush/images/bbb155instrubkcovpg5bbb.jpg
http://www.newstyleservice.com/155airbrush/images/bbb155instrubkcovpg6bbb.jpg
http://www.newstyleservice.com/155airbrush/images/bbb155instrubkcovpg7.jpg
http://www.newstyleservice.com/155airbrush/images/bbbackpageallinstrubksbb.jpg
1. The screws in question look to be an adjustment of the needle, presumably so that the needle is closed in the tip when the trigger is forward. This should be preset from the factory, so you probably shouldn't need to adjust it in the near term.
2. It's hard to give a good answer to thinning ratios because paints can vary so much, even the exact same color from the same manufacturer can have huge sample to sample variations, such that one bottle may be perfectly good straight without thinning, and another bottle may need to be cut 2:1 or even 3:1. Getting the right thinning ratio is one aspect of airbrushing that takes some practice to get right consistently. It is said that you should shoot for the consistency of milk. How you judge whether or not you paint is at that consistancy is something you get with experience.
3. Although you didn't ask about air pressure, if your compressor has a regulator, you'll probably find that it works well at 15 PSI, although you might find yourself varying that anywhere from 10 to 20 PSI.
Basically the deal with the AB is that it will take some practice to get the hang of it. How thin is the paint, what is the air pressure, how close is the AB to the model, what speed are you moving over the model, what is your trigger position, all these things combine in sometimes not obvious ways to make a good paint job. Practice, practice, practice!
good luck with the new Badger,
Andy