Air conditioning compressor

I already have the compressor but my car doesn´t cool very well. actually i notice that there is a missing part on the upper part of the compressor like if something has to be conected there. but there isn´t. also the air contioning works only in full position. I bought this car used so I don´t know what parts are missing or how they are. please would you send me pictures too.
thanks, sorry for my english I´m just studying i live in costa rica central america so i speak spanish.