The part you call B is the dovetail bracket for the finder. This is just to allow the finder to be easily removed and replaced. It also allows you to easily swap in other finders.
The finder will be adjusted with screws on the part you call A. As I say, mine has both locking and adjustment screws. This was all described on a piece of paper that came with the finder.
Daylight is the ideal time to experiment with the finder. In this way you can easily see what you are doing. Aim the telescope at a distant object (like a mountain) and align the finder. You have to be familiar with your equipment by the time you get it under a dark sky.
500x is way too high a magnification. I wouldn't get an eyepiece that goes beyond 200x until you are really familiar with your setup. You will only be able to exceed 200x on rare nights of exceptional seeing.
My workhorse planetary eyepiece is a 15mm that provides 133x on my scope. This is great for general viewing of Jupiter and Saturn. I only go to 200x if the seeing can support it. As I said, Mars is getting smaller at the moment and you need to go a little higher (150x to 200x) to see much on it.
I'm sure you will be able to get pleasing views of Jupiter and Saturn with the eyepieces you have. Mars is a more challenging target.
Hope you are enjoying the new scope
Have fun,
Doug