Hello, I'm Jason.I will do my best to answer your question quickly and accurately using the information you have provided.
There isn't a true "radiator" cap, but there is an expansion (coolant/antifreeze) tank to the right (passenger) side of the engine compartment where coolant is added. It is near the rear of the engine compartment, in the corner.
Well, that tank supplies the radiator. There are markings on the outside of the tank that show the full point. If you're just topping the system off for regular maintenance, it shouldn't take a large amount. If you've just replaced a cooling system component, and drained the system, fill it until it's full, leave the cap off, and start the car. Keep filling until it doesn't take any more coolant, and cap it.
Your question asked was where to pour coolant, which I answered.
Trying to diagnose a possible head gasket leak, or cylinder liner problem, or other diagostic-instensive hands-on problem is beyond the scope of this format.