low coolant warning light is on during warm-up

[luckyron1]

2000 lxi w/500 hp D-60. I just had the coolant flushed and refilled in
the spring of last yr. I've looked under the engine shroud and can't
figure out where to add a 50/50 solution of diesel antifreeze/water. I
don't see a plastic radiater overflow/reservoir tank or what you'd
expect to see under the hood of a car and for that reason I don't even
know how much to add. Where do I put the the fluid in and how much do
I put in? Thanks.