If youi can't find it, how do you know it's the radiator? If you can't find the leak when it's well heated up and pressurized, a couple of possibilities come to mind. One is that it isn't pressurized, and the coolant loss is from a bad radiator cap. Another is that it's leaking internally from a bad head gasket (hope not!). If you're sure it's not one of those things, your best bet is to find someone who has a radiator pressure tester, and pump it up and look for the leak that way.