this may be a bit late, but the low side is after the evaporator, follow the lines from the compressor, if one heads to the condenser (radiator looking thing in front of car) then thats the high side... you may want to have your low pressure switch checked before you add refrigerant, because too much will cause it to blow warm. also if you can, have a dye placed in the system if you do add fluid so you may find a leak later.