I have broken the little thingy in the back twice. Is it some sort of spring?
No- that may be why you are having problems.
Turn it around, and on the back plate are several screws-- ignore all but one, different from the others, which is inside a little hole in the plate. That is the adjusting screw.
All the other screws mount the innards to the plate, and should not move easily-- the recessed adjustment screw should move easily, and when it does, the dial reading will go up or down according to the direction of rotation.
I haven't done this since being a teenager, setting the barometers on Dad's boats, but it can't have changed that much!
PS- here is a link to a more detailed description regarding barometer adjustment:
http://www.windandweather.com/store/technical_faq?Args=