where is the low side tube to add freon

First question: What testing have you performed to determine your system is "low"? I find through this forum that way too many folks are working with air conditioner refrigerant, in this case R134a (which is NOT "freon", sorry to be picky), when they have no business doing it due to the hazards to themselves and very importantly - the atmosphere! Venting refrigerant to the atmosphere, even accidentally, is unlawful. A person asking where the "low side tube" is located simply is not at all qualified to be doing this repair. Further, it's highly irresponsible since you can't possibly have the equipment necessary to recover the refrigerant, vacuum test the system for leaks, test the system electrically -- checking the compressor coil and clutch, the cycle switch and other components to see what the real trouble is. I'm sorry if I sound preachy, but this is serious business, and not a "backyard mechanic" activity. Seek professional guidance.