Okay...since you're a novice at mechanics, let's start with the basics. Your radiator serves two purposes...1) obviously, it allows for the cooling of engine coolant but also, 2) has coils contained within it that also allows for the cooling of transmission fluid (the red fluid you're seeing on the bottom of the radiator). The two fluids are not normally in contact with each other. Thus, in addition to the familiar two larger radiator hoses, you'll notice there are two metal tubes/lines leading to it on the driver side (transmission fluid).
Now, if you're seeing red fluid on the bottom of the radiator, that would indicate a transmission fluid leak at the radiator. And, it would have nothing to do with the engine coolant being low. The only place I can think of that would cause an external leak at the radiator is from one of the metal transmission lines running to the radiator. If there were an internal leak, it would mix with the engine coolant and remain contained within the radiator/engine/transmission (btw...that's not a good situation).
Your low engine coolant level is a separate issue. Could be as simple as the radiator hose clamps need tightening. I've also had leaks around the fittings at the radiator where the metal transmission tubes connect to the radiator. Wipe the areas dry and watch for fresh leaks after you drive it again. If not there, I'd be looking at a cracked radiator housing (side tanks are plastic and prone to leaking with age), thermostat housing, heater hoses, leaky heater core (you'd probably be smelling coolant inside the truck and/or have excessive condensation on the windows or wet carpet) or worse yet...bad head gasket.
