If the compressor is running and it's not cooling it's most likely low on freon. It could be a bad compressor but it's not easy to tell. If it's 134a coolant, you can get a cheap set of gauges at harbor freight and a can a freon from the auto store. Most new systems are sealed and you will need a piercing valve like mentioned above to install the freon. I would not install any stop leak, it could just be a very small leak. Charge the system and see how long it goes.