I just bought this bsa deerhunter with the rear adjustment. I have never sighted in one before. What is the best way to sight in this type of scope and what is the rear adjustment for?

For magnified scopes and plain old red dot sights, there are usually two dials you adjust - one for up and down, one for left and right.
My ACOG replica and BSA red dot sights require a small flat head to be able to adjust said dials. Most sights follow the same thing, aside from holo sights which have buttons that don't require tools.

Medium height 1"tube 9-11 mm dovetail mounts

Try chambers gunsmiths if in UK or John knibbs international

Need more info.

adjust? what do you mean?
adjust what?
there are 2 adjustment knobs on it right?

To raise the point of impact, turn the elevation screw counterclockwise. To shift left, turn windage screw clockwise. If you raise the horizontal cross hair you will lower the point of impact of the bullet. If you move the vertical cross hair to the right the shot placement will go to the left. Make small adjustments tap the scope with a screwdriver handle to give it a shock, not to hard to help move the cross hairs and try to bore sight first. Hope this helps Fix ya up.

According to their website, the adjustment knobs have a "Push-Pull" feature. It appears that you need to pull the knobs away from the tube to engage the adjustment mechanism. Once you have accomplished this, you should be able to adjust you Point Of Aim (POI), then push them back in to prevent inadvertent setting changes.

The reverse may also be true - pressing in to change the setting, pulling back out to lock in the adjustment.

Hope this helps

What that means is they are not mounted correctly. The scope mounts should be square with your rifle when fitted, when the scope is placed into the mounts, it should be turned to be square with the rifle.

Firstly, make sure you do not "cant" when you shoot. That is hold the rifle at an angle instead of vertical. Once you are sure that is OK, then put it into a vice using blanket or some other soft material to protect the stock. Get the rifle perfectly square using a spirit level. There should be a flat spot on the top of the action that you can use. Once this is achieved, you loosen off the scope mounts and square the scope up to your satisfaction. Once done, tighten the mounts up and then remove from the vice.

Go to; www.bsaoptics.com for the manual and parts.
You do not say if the WINDAGE Adjustment KNOB is 'broken' or what? If you have dropped it or somehow broken it, it's unlikely that you will be able to repair it yourself, in which case I refer you again to BSA.
If there's nothing wrong with the scope or your rifle, and you are simply having difficulty getting a good ZERO, I can help you with that, just respond and let me know.
Good luck. :)