The orifice tube is usually next to the evaporator inside the car, and is used in lieu of a TXV to provide the pressure drop necessary for proper A/C operation.
However, your problem is a leak in the system somewhere, and trying to just throw some freon in will likely just cause more problems, like frozen lines.
Take it to a shop that can locate the leak, repair it, evacuate the system properly and put the correct amount of charge in it. Yes it will be costly, but if you want the A/C to work properly, it is the only way to do it right...
