Coolant problem

Hi,

I am having trouble with the coolant. It was full but after driving about 150 miles, the tank was empty. I can't get any coolant now and need to move the car. So I am planning to put some water in the system through the radiator cap. But I can't find it. I hear it is usually in the front near the radiator but I guess Beetle is built differently. I tried to put water through the overflow reservoir with the engine run but it boils over and still have the red overheated coolant light on. Can someone tell me where the radiator cap is?
Thanks.

The coolant overflow bottle is the correct way to add coolant. Sounds like you have a blown head gasket unfortunately.